doerrzangor2000creatingmeaningforandwiththegraphingcalculator

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Creating Meaning for and with the Graphing Calculator

Helen M. Doerr; Roxana Zangor

Educational Studies in Mathematics, Vol. 41, No. 2. (Feb., 2000), pp. 143-163.

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HELEN M . DOERR and ROXANA ZANGOR

CREATING MEA NING FOR AND WITH TH E GRAPHING CALCULATOR

ABST RACT . In this study, we seek to describe how the meaning of a tool was co-constructed by the students and their teacher and how the students used the tool to con-struct mathe matic al meaning out of particular tasks. We report the results of a qualitative.classroom-based study that examined (1 ) the role, knowledge and beliefs of a pre-calculusteacher, (2) how students used graphing calculators in support of their learning of math-ema tics, (3) the relationship and interactions between the teacher's role, knowledge andbeliefs and the students' use of the graphing calculator in learning mathematics, and (4)som e constraints of the gra phing calculator technology that emerged within the classroompractice. We fhund five patterns and modes of graphing calculator tool use emerged inthis practice: computational tool, transformational tool, data collection and analysis tool.visualizing tool, an d checking tool. The results of this study suggest that nature of the math-ema tical tasks and the role, knowledge and beliefs of the teacher influenced the emerg enceof such rich usage of the graphing calculator. We also found that the use of the calculatoras a personal device can inhibit communicatio n in a small group setting. while its use as ashare d device supported ma them atical learning in the whole class setting.KEY WORDS: graphing calculator, tools, pre-calculus, technology. teacher knowledge

Functions and graphs have been the focus of numerous research studiesover the past decade. The study of students' understanding of the conceptof function, and their abilities to create and interpret graphical representa-tions, was given strong impetus by the advent of computers and their readyavailability in som e classrooms. This led to many computer-based studiesthat analyzed students' reasoning w ith and about linked, dynamic multiplerepresentations of functions (e.g., Confrey and Doerr, 1996; Leinhardt,Zaslavsky and Stein, 1990; Moschkovich, Schoenfeld and Arcavi, 1993;Yerushalmy, 1991). Yet despite the limitations of the graphing calculator


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144 H.M. DOERR AND R. ZANGOROver the past decade, secondary mathematics teachers have moved to

adapt the graphing calculator into their practice. The National Council ofTeachers of Mathematics's (N CT M ) curriculum standards (NC TM , 1989 )recommend using the graphing calculators to provide students with newapproaches, such as the use of multiple representations, to the investigationof mathematical ideas. While it might appear that practice has moved in-dependently of and more quickly than research, there is in fact no shortageof research studies on the use of the graphing calculator. See Dunham andDick ( 19 94 ), Wilson and K rapfl (199 4), and Penglase and Arnold (1 996)for extensive reviews. Many, if not most, graphing calculator studies arequasi-experim ental in design and seek to answer the question of whether ornot graphing calculators are effective in achieving certain instructional ob-jectives, which are often left unchanged from traditional paper-and-pencilapproaches (Adams, 1997; Quesada and Maxwell, 1994; Ruthven, 1990:Sho af-Grub bs, 1994). Many such studies compare the use of the graph-ing calculator to the use of paper and pencil on the same set of tasks,giving only limited insight into how and why students use graphing cal-culators in the instructional context. One such insight is provided in thework of Ruthven (1990 ), who found that students developed three distinctapproaches to symbolizing a graph: an analytic-construction approach thatexploits mathematical knowledge, a graphic-trial approach that comparessuccessive expression graphs with the given graph, and a numeric-trialapproach guided by the coordinates of the given graph.

The relationship between teachers' knowledge and pedagogical strate-gies and their use of the graphing calculator is largely unexamined. Manystudies (Drijvers and Doorman , 1996; Hollar and Norw ood , 1999; Porzio,1997) do not repo rt or describe the role of the teacher in the classroomor the teachers' knowledge and skill with the graphing calculator or theteachers' beliefs about the efficacy of or hnds of uses of graphing calcu-lators in mathematics learning. In their review of the research, Wilson andKrapfl (1994) suggest that there is a need to better understand teachers'conceptions and a need 'to focus on the qualitative aspects of knowledgeconstruction of students using graphing calculators' (p . 261). As Penglaseand Arnold (1996) point out in their extensive review of the literature,many studies fail to distinguish carefully between the tool and the in-structional context and assessment procedures, leading to inconsistent find-


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CREATING ME ANING FOR AND WITH THE GRA PHING CALCULATOR 145overall shift in teachers' roles from task setter and explainer to consult-ant, fellow investigator, and resource. These shifts occurred within eachclass as the teachers moved from not using the graphing calculator andcomputer technology to the use of it; the shifts were also accompaniedby a decrease in lecture and an increase in group work. Contrary to thesefindings, other research has found that teachers did not change their meth-ods or approaches when using the graphing calculator in teaching abouttransformations of the parabola, except to provide more visual examples(Sim mt, 1997). The teachers in Sim mt's study did not use the graphingcalculator to facilitate discussion, encourage students to conjecture or toprove ideas. The finding that these teachers did not shift their role whenusing the graphing calculator may be accounted for in part by the shortperiod of time they used the technology. 4 to 10 class periods (Simmt.1997, p. 273) . The teachers in Farrell's study (w ho had shifted their roles)were observed using the graphing calculator near the end of a year longcourse. However, the more important variable in influencing the teacher'srole with the graphing calculator may well have been the teachers' attitudesand beliefs about mathematics and mathematics education.

This relationship was explored in a study by Tharp, Fitzsimmons andAyers (1997) who found that teachers who held a less rule-based view-point about learning mathematics were more willing to adopt the use ofcalculators as an integral part of instruction than those teachers who held arule-based view of mathematics learning. These researchers found that thenon-rule-based teachers used more inquiry learning in their classrooms andthat their students freely used the calculators as they wished. The teachersin Simmt's (1997) study were described as having a view of mathematicsthat 'strongly favors 'traditional' approaches to problems' (p. 287), sug-gesting that they may be more rule-based and less likely to adopt a more in-tegrated use of graphing calculator technology. The findings of Tharp et al.suggest that there is an important relationship anlong the teacher's know-ledge and beliefs, pedagogical strategies, and how students use graphingcalculators.

In this paper, we report the results of a qualitative, classroom-basedstudy that examined (1) the role, knowledge and beliefs of a pre-calculusteacher, (2) how students used graphing calculators in support of theirlearning of mathematics. (3) the relationship and interactions between the


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our findings on how the graphing calculator, as with any tool o r technology,enabled and constrained both pedagogical practices and student learning.

3. M E T H O D O L O G YN D D A TA A N A L Y S I SA pre-calculus curriculum based on modeling problems in an enhancedtechnology environment (using graphing calculators, calculator-based meas-urement probes for motion, temperature and pressure, and computer soft-ware) provided a rich setting for studying the role of the teacher and thepatterns and modes of graphing calculator use by the students and theteacher. This classroom -based, observational ca se study of two pre-calculusclasses took place in a suburban school. The classes were taught by thesame teacher and each met for five sessions for a total of 270 minutes perweek. One class had 17 students and the other 14 students, all between15 and 17 years of age. This selection of a single teacher in two similarclasses allowed us to focus in-depth on the role and knowledge of theteacher and the interactions with and among the students. The teacherhad 20 years of teaching experience and was skilled in the use of thegraphing calculator. The classes were observed over three units of studyon linear functions (four weeks), exponential functions (ten weeks) andtrigonometric functions (seven weeks).

All of the students had either TI-82 or TI-83 graphing calculators. Thesedevices are rich in graphing and statistical functionality, although lack-ing in symbolic algebra capability. In general, the students had used theircalculators for well over a year before taking this course and were quitefamiliar with its functions. The classroom w as equipped with a com puter,printer and a 'graph link cable' that could be used to transfer pictures ofthe calculator graph to the computer for printing. T he link cable also couldbe used to transfer data and programs between the computer and calcu-lator, but this feature was rarely used. On the other hand, students readilytransferred data and programs between calculators using the calculator-to-calculato r link cable. Th e classroom was equ ipped with a view screen thatallowed the calculator screen (but not the keystrokes) to be projected usinga standard overhead projection unit.

Classroom instructional activities regularly alternated between model-


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148 H.M . DOERR A N D R. ZANGORasked to interpret data, to find mean ingful representations of the data (typ-ically tables, graphs and eq uations), and to generalize relationships beyondthe particular situation at hand. Th e con cep t of the rate of change of a func-tion and the transformations of exponential and trigonometric functionswere addressed throughou t the problem situations.

All class sessions were observed by two or more members of the re-search team. Extensive field notes, transcriptions of audio-taped groupwork, transcriptions of video-taped whole class discussion, and interviewsand planning sessions with the teacher constituted the data corpus for thisstudy. In an observational study such as this, there are necessarily somedifficulties in observing students' use of the graphing calculator. As Wil-liams (1993) has pointed out, it is not easy to see the students' calculatorscreens, much less to capture the keystrokes they are using. However, themathematical investigations were specifically designed to be co mp leted bysmall groups of students and then later shared in a whole class discussion.This format generated discussion within the small groups where studentsexplained to each other what they did on their calculator, or one studentwould show another how to do a particular task, or students w ould com paregraphical or numerical results. In this way, students' calculator use wasmade visible to the researchers. However. there were occasions where itsimply was not clear what the students had done with their calculators. Th euse of the graphing calculator in the whole class setting was much easierto observe, since the teacher and the students regularly used the overheadprojection view screen and explained what they were doing as they weredoing it.

Th e data analysis was com pleted in tw o phases. In the first phase, thesedata were analyzed and coded for the patterns and modes of graphingcalculator use by both the teacher and the students throughout the instruc-tional units. Upon completion of this initial coding, a descriptive profileof the teacher. including her knowledge, role in the class, and beliefs wascompiled. This profile was shared with the teacher for her corroborationand feedback. The initial coding of the data on the patterns and modesof student use led to the creation of five categories of graphing calculatoruse througho ut the three instructional units. The few discrepancies in cod-ing between the researchers were resolved by comparing the episode inquestion to similarly coded episodes and to other episodes involving the


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light of her feedback. Since the teacher substantially corroborated the pro-file, this led to only minor changes in that profile. With our refined setof categories of graphing calculator use, we sough t to identify limitationsthat the technology im posed o n the teachers' pedagogical practices and onthe students' use of the tool to support their learning. This phase of theanalysis yielded a detailed description of the limitations and constraints ofthe tool. We then completed our analysis by identifying the links betweenthe teacher's role, knowledge and beliefs and the students' patterns andmod es of calculator use.

We begin with a brief description of the role, knowledge and beliefs ofthe teacher as they were reflected in her pedagogical strategies and in herinteractions with the stud ents. We then present the patterns and m odes ofgraphing calculator use that emerged as the students interacted with theteacher, with each o ther and w ith the m athem atical tasks. Finally, we d is-cuss ou r findings a s to how the graph ing calculator en abled and constrainedthe students' learning and the teacher's pedagogical practice.4.1. The reacherThe teacher was particularly skilled in using the graphing calculator, aswas demon strated throughout the instructional units by her ow n use of thecalculator and by her ease in answering the studen ts' occasional questionson a particular calculator procedure. The teacher was familiar with theprogramming features of the graphing calculator and she had written ashort program that would calculate the average slope from an ordered listof ordered pairs of data. She felt comfortable reading programs that thestudents would occasionally write. The teacher had also devised her ownmethod for finding a numerical approximation of derivative of any givenfunction. Since this was a pre-calculus course, she did not want to useany 'black box' or built in functions of the graphing calculator to find thederivative, but rather she wanted to m ake the creation of the rate of changefunction explicitly visible to the students through the use of the familiar


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150 H.M. DOERR AN D R. ZANGORcussions . The teacher herself showed flexibility in her use of the calculator.While using the overhead projection unit, if a student suggested an altern-ative viewing window or another section of a table to examine or a similarexpression for comparison, the teacher was willing to take the student'ssuggestion.Th e teacher believed that the calculator presented certain mathematicallimitations. She raised the issue of the validity of the calcu lator results byasking questions such as 'Does the calculator always tell the truth?' and'To what extent should we believe the calculator?' For example, in onetask, the students were investigating a decay situation where they beganwith a small cup full of small, disk-shaped candies with an 'M' on oneside. Th e candies were spilled onto a table and those with the 'M ' showingwere removed on each trial until none rem ained. (See Doerr (1998) for afuller description of this mathematical task.) The students concluded thatan exponential function models this decay process. One student observedthat even though in their experiment they ended up with zero candies, theexponential model did not attain a zero value since 'you can divide by twoinfinitely without getting ze ro'. But ano ther student, who was manipulatingthe calculator on the overhead screen, scrolled down the table for theirexponential function until, for very large values of x,the function appearedto reach the value zero. This generated considerable discussion among thestudents about the calculator having limitations and 'not always telling thetruth'. The students began to see the calculator as a tool that should bechecked based on their ow n understandings of mathem atical results.The teacher believed that the calculator would be a helpful tool for thestudents to use in finding meaningful responses to problem situations. Twospecific calculator-based methods (regression analysis and curve fitting bymodifying parameters) were regularly used by three of the 3 1 students tosolve problems where part of the task was to find an equation of a functionto represent the data set for a given phenomena. These tw o methods did notbecome very popular among the students as a whole, but rather remainedin use extensively by just three students. The teacher did not explicitlydiscourage these methods by telling the students not to use them, but rathershe required a meaningfu l explanation of how the num erical results relatedto the problem situation. This meaningful interpretation of the result couldnot be given by the students who used either regression analysis or a curve


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CREATING MEANING FOR AND WITH TH E GRAPHING CALCULATOR 151TABLE I

Patterns and modes of graphing calculator use Role of the Graphing Calculator Description of Student ActionsComp utational Tool evaluating numerical expressions, estimating

and roundingTransformational Tool changing the nature of the taskData Collection and Ana lysis Tool gathering data. controlling phenomena, finding

patternsVisualizing Tool finding symbo lic functions, displaying data, in-

terpreting data, solving equationsChecking Tool confirming conjectures, understanding multiplesymbolic forms

the result's meaning through a mo re mathem atical analysis of the problemsituation. Th e teacher's belief that the calculato r cou ld be a helpful tool tofind meaningful representations of problem situations led to a de-valuingof the regression coefficients that were easily generated by the calculatorand of curve fitting by modification of parameters.4.2. The stzidentsThrough our analysis of the data, we identified five categories of patternsand modes of calculator use by the students (see Table I) . The focus ofour analysis is on how and why these features were used in the problemcontexts an d instructional situations. T he g raphing calculator can in certainsituations take on m ore than one role simultaneously.

We will carefully de scribe each of these roles of the graphing calculatorand its relationship to the teacher's role, knowledge and beliefs. We willthen present the findings on the limitations and constraints of the graphingcalculator in this particular teaching and learning environm ent.4.3. The calculator as a computational toolAs a com putational tool, the graph ing calculator was routinely used by stu-dents to evaluate numerical expressions . Two com putational issues arose:


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152 H.M. DOERR AND R . ZANGORfunction and to the accepted ma thema tical fact that the logarithm of a neg-ative number does not exist. We interpret that this call for an explanationfor calculator results (whethe r errors or not) had becom e part of the math-ematical norms established by the teacher, whereby the accepted truth orfalsehood of a statement had to be supported by mathematical reasoning orjustification, not by an appeal to any authority ascribed to the calculator.A second computational issue emerged as the students attended to theneed for rounding (or approximating) the results given by the calculator.The need for an approximation of the numerical results given by the cal-culator was closely linked to the interpretation the students gave to theproblem situation. Their interpretation of the constraints and restrictionsimposed by the context of the problem determined whether and how theysaw the range of the possible numerical values of the result. This happened ,for example, in recursive discrete processes of exponential growth anddecay. One group of students discussed whether to round the result at eachstage or at the end of the process in a population growth problem. Theyargued that the fractions at each stage could add up to an integer at the endand that therefore this could affect the final size of the population. For pricediscount problems. the students relied on their common sense knowledgeof store prices and rounded the discounted prices to the nearest dollar, butonly at specified time intervals. They knew that a discount would not bemade daily, but more likely at the end of each week. The mathematicalmeanings of the numerical results were interpreted by the students so as tomake descriptive sense of the experienced phenom ena.

While the students attended to the issue of rounding in the problemsituations just described, there were other situations in which the studentssimply worked with whatever number of decimal places the calculatorhappened to provide. For example, the students readily worked with num-bers having many decimal places that represented measurements for dis-tances generated by a motion detector, water temperatures measured bya temperature probe, pressure sensed by a pressure belt and meter, andheights calculated using the sine function. The students would record andcalculate with these numbers, even though they appeared to recognize that'real-life' measurements could not be taken to 6 or 8 decimal places ofaccuracy. They showed no tendency to round the numbers and providemore 'realistic' answ ers. O n the contrary, the large num ber of decimal


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formed into interpretative tasks. As we noted earlier in our profile of theteacher, this teacher regularly asked students about the meaning of thecoefficients and constants in any equations they used. By her focusing thestudents' attention on the interpretation of the result, rather than on theactual computation, the students attended to making sense of the resultand validating it in the context of the problem situation. This transform-ation of tasks from computational to interpretative was especially evidentthroughout our analysis of the students' investigations of several rate ofchange problems.

As a computational task, determining the rate of change for a givenfunction for small variations of the independent variable on the entire do-main would be tedious to perform without a calculator. Using a paper andpencil approach for this task would restrict the study of the rate of changeto a local property instead of the global view of the rate of change, whichin turn leads to an understanding of the rate of change as a function itself.Many students used their graphing calculator to construct 'rate functions'of the form p(x) = [y, x+Ax)-y1 (x)]/(Ax), where y1 (x) s the originalfunction and Ax s an interval for the independent variable. The studentsusually set this value to . I or .01.This syntax for the rate function had beendevised by the teacher and introduced to the students through the use ofthe familiar formula for slope. This form was initially used with standardfunction notation and then became instantiated in the calculator notationgiven above.

The students and the teacher continued to use both paper-and-penciland the calculator for estimating the rate of change of a function . However,it was the calculator-based form of the numerical estimate of the rate ofchange of a function that transformed tasks from a computational focusto an interpretative one. For example, the students were given the task offinding a function to describe the vertical position of a point on a rotatingFerris Wheel. relative to the hub of the Ferris Wheel. The students werealso asked to find a function to describe the corresponding rate of change inthe vertical position with respect to time. Nearly all groups of students usedthe 'rate function' to construct the rate of change function, j 2 , after theyhad found a suitable expression for the vertical position function, JI.Thiscomputation of the rate function generated the table of values for the rate-function and its corresponding graph . From this, they were able to validate


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154 H.M. DOERR AND R. ZANGORdetails of each pairwise com putation for rate to a global interpretation ofthe function over its dom ain and in relationship to the problem's context.4.5. The calculator as a data collection and analysis toolDuring activities with Calculator Based Lab (CB L) devices, motion detect-ors, pressure belts, temperature probes, and special programs, the calcu-lator was used as a tool for data collection and analysis. The co llected datawas stored, compared, and re-collected until the students decided that theyhad acquired a 'satisfactory' data set. Through this kind of use of the tool,students needed to engage in understanding the context of the activity andin deciding, through a process of conjecturing, refining, and negotiating,what constituted a 'satisfactory' set of data. In several of these activities,we found that students repeated the same experiment many times, untilthe set of data they had collected graphically matched an expectation theyhad about the behavior of the phenomena. There were two distinct modesof repetition that emerged. O n the one hand, in working with temperatureprobes and the motion of a bouncing ball, the students acted with relat-ively little direct control over the gathering of the data. In this case, thestudents would simply repeat an experiment hoping to acquire a data setthat more nearly matched their expectations. On the other hand, there wereseveral activities that evoked a much greater sense of direct control overthe phenomena.

In one such activity, the students were asked to use a pressure belt togather data that represented the pattern of 'normal' breathing and to finda function that could be used to describe that pattern. Since many of thestudents expected to obtain a sine-type plot for their data, they purposefullyadjusted their breathing in terms of both the amount of air inhaled andthe rate at which it was inhaled. They attempted to control their breathingand hence the data collection device and the resulting data in such a waythat the data were closer to expected shape and numerical characteristics(i.e. without jagged peaks and almost constant amplitude). In this way,the graphing calculator and the attached pressure belt became a tool thatsupported the students in controlling the data collection through cyclesof interpretation of the graphical results and purposeful changes to thephysical phenomena of breathing.


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link the visual representation to the physical phenomena, and (4) to solveequations. Whenever the students had a set of data, they were quick toplot the data ; this inclination to visualize data could be seen throughout allthree units, but most especially in the exponential and trigonom etric units.A small number of students frequently used a visual parameter matchingstrategy when they were faced with the task of finding an equation thatdescribes a given data set. Th e approach taken is illustrated by on e studentin his attempts to find an equation for the harmonic motion of a spring,gathered from a motion detector. This student, basing his strategy on the re-cognition of the class of trigonometric functions and his knowledge aboutthe symbolic representations of transformations, started with an equationof the type y = A sin(B(x + C)) + D. This student systematically variedthe parameters A , B, C, and D in his equation in order to obtain a visualfit between the plot of the data set and the graph of the function .

The students became very skilled at determining appropriate viewingwindows for data sets and for symbolically defined functions. Sometimesthey used trial and error approaches to find better viewing windows, butmost of the time they set the window by using specific entries from thetable values of the relationship to be graphed. They developed the realiza-tion that a quick judgment based on the shape of a graph was not enoughfor deciding the nature of the function. The students had seen situationsin which an exponential shape can be visually mistaken for a quadraticshape and when a close zoom in on a function made it look linear. Asa consequence of this and of the teacher's belief that the calculator itselfcannot provide the authority for a mathematical argument, the studentsfound reasons other than appearance to justify the underlying structure ofthe function.

The graphing calculator was used as a visualizing tool when the stu-dents exam ined the discrete data values describing a phenomenon and hadto relate the data back to the continuous physical process and the func-tion which potentially describes that process. For example, in the FerrisWheel investigation, a group of students constructed a table for the rateof change using their 'rate function' and observed that what appearedto be the highest value of the function repeated consecutively. Knowing,from their experience of problem situation, that the motion of the wheelwas continuous, they decided that 'there was something in between'. The


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156 H.M. DOERR AND R . ZANGORdiscrete data table, which was missing a critical data point (the maximumvalue), and the physical phenomena, which, from their experience with thewheel, the students conjectured must reach som e unique maxim um value.

The graphing calculator's use as a visualizing tool was also reflectedin how the students solved equations or inequalities. In the class discus-sions, the teacher had discussed all of the available methods for solvingan equation and encouraged the students to choose among these methods.The methods included paper and pencil solutions, use of the calculator's'Solve' command, and graphical solutions. We found that many of thestudents used a graphical approach, which involved less computation andappeared to support a more meaningful interpretation of solutions. For ex-ample, in one problem, the students were asked to determine when a bankaccoun t attained a given amoun t of money. Many students approached thisby examining the intersection of the graph of the function describing theamount of money at any point in time w ith the graph of the horizontal linerepresenting the given amount. This visual approach seemed to supporttheir description of the amount of money in the bank reaching a givenvalue.4.7. The calculator as a checking toolIn this study, we considered the use of the calculator as a checlung toolwhen it was used to check conjectures made by students as they engagedwith the problem investigations. This was followed by rejecting and re-conjecturing, trying to prove the conjecture, or simply accepting the con-jecture. In many tasks, the students posed a conjecture about a possiblefunction as fitting a data set and then they used their graphing calculatorto check how w ell it fit. Th e students' strategies depended in part on howthey had found an equation in the first place. For example, in the case of thestudent using visual parameter matching for find an equation for the datafrom harmonic motion (described above), the visual mis-match betweenhis conjectured equation an d the data was crucial for his decision to rejecthis equation and try a new one . In this way, the g raphing calculator was anessential check of the match between his equation and the data. In othercases, we found that the calculator was used as a check ing tool only in themost trivial sense. For example, when one student found an equation using


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When the students had determined the function through a meaning-ful mathematical process, they would check to see how well their graphmatched the data. Occasionally, a graphical mismatch revealed a mistakein entering the form of the equation or in a computation. For example, infinding an equation for the harmonic m otion described earlier, one studentdetermined the amplitude, both the horizontal and vertical shift, and thefrequency by tracing the coordinates of consecutive maximum points inthe data set. This approach enabled her to find a meaningful equation basedon her knowledge of trigonometric functions and their transformations. Inthis case, the check between her conjectured equation and the data set wasconfirmed as she observed that the graph and the data were visually alignedalong critical features such as local maxima and minima.

The calculator's role as a checking tool was evident in the context ofactivities on transformations o n functions, which were designed both in go-ing from the equation to the graphical representation and from the graph tofinding an equation (non-unique) to match the given graph. The calculatorwas initially used as a visualizing or graphing tool, but as students graspedthe ideas of transformations, the calculator was used almost exclusively asa checking tool. The graphing calculator helped the students understandthe idea of the non-uniqueness of algebraic representations for the samegraph in the case of exponential and trigonometric functions. For exam ple,the students were asked to compare the graph of y = 8 * 2" with thegraph of J = 2'+3 and found that the graphs appeared identical. Using theoverhead d isplay unit, the teacher switched to the table of values, revealingthat the numerical values were also identical. This then led to an algebraicargument that supported the claim that these two functions were identical.In this way, the graphing calculator was used to confirm conjectures, but atthe same time, the teacher led the mathematical discussion to the need forjustification by algebraic reasoning.

The role of the graphing calculator as a checlung tool by both thestudents and the teacher was especially interesting in the case of the trigo-nometric functions. The periodicity of the trigonometric functions resultsin a particularly salient characteristic, namely the 'look alike' feature of thegraphs of these functions. One can argue that the 'look alike' appearanceon the screen of the calculator is comm on to almost any class of functions,certainly to the exponential class that was studied by these students. How-


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158 H.M. DOERR A N D R . ZANGOR

not labeled. The investigation of trigonometric functions with the calcu-lator became limited to the use of the tables and occasionally the 'Trace'command. This pattern of use as a checking tool for the trigonometricfunction was limited by a mismatch between the decimal representationsof the calculato r and many of the tasks where the values of the independen tvariable were often given as rational multiples of n.The calculator's tableand the 'Trace' com man d show ed these values in decim al form. Hence,there was no easy direct comparison of the closeness of numerical val-ues without an extra tedious conversion step of the value of the rationalmultiple of rr to a decimal value.4.8 . Constraints and limitations vvith the graphing calculatorThe graphing calculator emerged as a constraint and limitation in twoways: (1) students' attempted uses of the device as a 'black box' withoutattending to meaningful interpretations of the problem situation; and (2)the personal (or private) use of the tool. The use of the calculator as a'black box' represents the class of situations in which the students did nothave a meaningful strategy for the use of the calculator. In one particulartask, the students were asked to design simulation of the spread of a ru-mor. The students were given the calculator syntax for generating singlerandom numbers and lists of random numbers. Some groups of studentsdecided that they had to use the random number generator and tried todo so before they actually read the content of the activity and understoodwhat the random number syntax might be useful for. These students didnot understand that the calculator's random number generator was simplyan alternative tool, not the only tool, and certainly not 'the solution' to theproblem . At least one group of students became considerably sidetrackedon the quantitative details of how the random num ber worked. Th ey spentconsiderable time discussing if they needed to correct for whether or not azero could be generated by the random num ber generator. This discussionoccurred well before they had found a use for random numbers in thedesign of their simulation.

Another limitation of the graphing calculator was in its use as a privatedevice. Unlike the case of group work with a computer, where the screenis a visible and hence potentially shared artifact, the calculator's screen


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ations or refinements of their ow n thinking. The results of the graph, table,or computation were used to further their own individual thinking, ratherthan shared back with the group. This closed the communication networkamong the students and made the re-opening of discussion quite difficult.As time progressed, the students had different interpretations, represent-ations and problems rather than a common or shared view of the task athand. Once this closing of communication had happened within a group,the students tended to continue to pursue their investigations as individuals,with little talking and no sharing of ideas, representations, or results. If thestudents had questions, they would ask them of the teacher, rather thaneach other.This use of the calculator as a private, personal tool was in sharp con-trast to its use during the class discussion, as a shared device (via theoverhead screen). This shared, public display was supportive of the com-munication among students and was used to facilitate the comparison andunification of ideas. S ince the teacher let the students control the calculatorin most instances, this shared device helped encourage student initiativeand often resulted in the students' leading of the discussion. The decisionto use the calculator as a shared device in the classroom came from thedevelopment of the discussion and was often student-requested.

The role, knowledge and beliefs of the teacher and the actions of the stu-dents as they interacted with each other and with the graphing calculatorcreated meaning for the tool and meaning with the tool. The teacher'sconfidence in her own knowledge and skills and her own flexible use ofthe calculator led to a classroom environment where students were freeto use their calculators as they wanted and were actively encouraged touse them to calculate, explore, confirm, or check mathematical ideas. Theteacher's knowledge of the limitations of the calculator led her to encour-age the students to question their calculator-based results. In this way, thecalculator became a tool that itself needed to be checked on the basisof mathematical reasoning. Contrary to the concerns raised by Williams(1993) and Wilson and Krapfl (1994), the calculator did not become a


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160 H.M. DOERR AND R. ZANGORtasks is evidenced by the kinds of questions she asked the students. Sheregularly asked the studen ts about the meaning of the coefficients that theyfound and how those coefficients related back to the experienced phenom-ena . We found that in finding equations to fit graphs o r numerical data sets,the most com mo n strategy used by the students was a meaningful mathem-atical approach. The visual-parameter matching strategy, while based onthe mathem atical recognition of the general form of the function, was usedextensively, but by only a few students. T hese findings confirm the resultsfound by Ruthven (1990). Furthermore, for the trigonometric functions,we found that the use of the graphing calculato r as a checking tool to con -firm their conjectures was not used by most students, but rather they reliedsolely on their algebraic and geometric knowledge of the transformations.

Th e teacher's belief in meaning m aking led to the de-valuing of regres-sion equations as solutions and limited the 'black box' use of the graphingcalculator. This was particularly visible in the teacher's strategic choice touse a rate function that was built upon the students understanding of thefamiliar slope formula rather than a use of built-in derivative functions.The rate function served to illuminate a powerful role for the graphingcalculator as transforming a local, computational task (the computation ofaverage rate of change) to a global, interpretative task (the relationshipbetween the numerically determined rate of change function and the givenfunction over the entire domain).

The role of the graphing calculator as a device that can both monitorand control data collection appears particularly powerful and has beenlittle explored in the research literature to date. In this study, we fo und thatstudents engaged in two distinct actions: one where they repeated the datacollection to match their expectations and the other w here they controlledtheir actions to match their expectations. As a visualizing tool, the studentsused the graphing calculator to find equations that matched data sets, tofind appropriate views of the graph, to link the visual representation of thegraph to physical phen omena in explanatory ways, and to solve equation s.The most significant limitation of the calculator was as its use as aprivate device. While w e did observe, as did Farrell (1996), that studentsfrequently used their calculators while the teacher or other students weretalking in lecture or whole class discussion, we also observed that this per-sonal use of the technology served to breakdown group communications.


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and K. Newstead (eds.). Proceedings of the 22nd Co~lferenceof the Internatio~~nlGroup for the Psychology ofMathemcrtics Education, Vol. 2, University of Stellenbosch.Stellenbosch, Africa, pp. 256-263.Doerr, H.M. and Tripp, J.: 1999, 'Understanding how students develop mathematicalmodels'. Mathematical Thinking and Learning 1(3), 2 3 1-254.

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Leinhardt. G., Zaslavsky, 0 . and Stein. M.K .: 1990. 'Functions, graphs. and graphing:Tasks. learning and teaching'. Rel,ie\r c!f'Educatiorlul Resecirch 60(1), 1-64.Meira. L.: 1995, 'Mediation by tools in the mathematics classroom', in L. Meira andD. Carraher (eds.). Proceeditig.~cf the 19th Annual Corferen ce of the InternationalGroups ,for the Psxchology of Mathematics Educa tion, Vol. 1. ERIC Clearinghouse.Columbus. OH. pp. 102-1 11.

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HELEN M . DOERR and ROXANA Z ANGORDepartment of MathematicsSyracuse Univ ersihSyracuse, Ne w YorkU.S.A. E-m ail: h m c lo er r @ s j~du


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You have printed the following article:

Creating Meaning for and with the Graphing Calculator

Helen M. Doerr; Roxana Zangor

Educational Studies in Mathematics, Vol. 41, No. 2. (Feb., 2000), pp. 143-163.

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References

The Effects of a Graphing-Approach Intermediate Algebra Curriculum on Students'Understanding of Function

Jeannie C. Hollar; Karen Norwood

Journal for Research in Mathematics Education, Vol. 30, No. 2. (Mar., 1999), pp. 220-226.

Stable URL:

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Functions, Graphs, and Graphing: Tasks, Learning, and Teaching

Gaea Leinhardt; Orit Zaslavsky; Mary Kay Stein

Review of Educational Research, Vol. 60, No. 1. (Spring, 1990), pp. 1-64.

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Making Sense of Instructional Devices: The Emergence of Transparency in Mathematical

ActivityLuciano Meira

Journal for Research in Mathematics Education, Vol. 29, No. 2. (Mar., 1998), pp. 121-142.

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