Chapter 2

Graphing Linear Relations

and Functions

By Kathryn Valle

2-1 Relations and Functions

• A set of ordered pairs forms a relation.– Example: {(2, 4) (0, 3) (4, -2) (-1, -8)}

• The domain is the set of all the first coordinates (x-coordinate) and the range is the set of all the second coordinates (y-coordinate).– Example: domain: {2, 0, 4, -1} and range: {4, 3, -2, -8}

• Mapping shows how each member of the domain and range are paired.– Example: 1 9 4 3

-3 2 1 0 7 -5 -2 -

67

2-1 Relations and Functions (cont.)

• A function is a relation where an element from the domain is paired with only one element from the range.– Example (from mapping example): The first is

a function, but the second is not because the 1 is paired with both the 3 and the 0.

• If you can draw a vertical line everywhere through the graph of a relation and that line only intersects the graph at one point, then you have a function.

2-1 Relations and Functions (cont.)

• A discrete function consists of individual points that are not connected.

• When the domain of a function can be graphed with a smooth line or curve, then the function is called continuous.

2-1 Practice

1. Find the domain and range of the following:a. {(3, 6) (-1, 5) (0, -2)}b. {(4, 1) (1, 0) (3, 1) (1, -2)}

2. Are the following functions? If yes, are they discrete or continuous?

a. {(2, 2) (3, 6) (-2, 0) (0, 5)} c. y = 8x2 + 4b. {(9, 3) (8, -1) (9, 0) (9, 1) (0, -4)}

Answers: 1)a) domain: {3, -1, 0} range: {6, 5, -2} b) domain: {4, 1, 3} range: {1, 0, -2} 2)a) function; discrete b) not a function c) function; continuous

2-2 Linear Equations

• A linear equation is an equation whose graph is a straight line. The standard form of a linear equation is: Ax + By = C, where A, B, and C are all integers and A and B cannot both be 0.

• Linear functions have the form f(x) = mx + b, where m and b are real numbers.

• A constant function has a graph that is a straight, horizontal line. The equation has the form f(x) = b

2-2 Linear Equations (cont.)

• The point on the graph where the line crosses the y-axis is called the y-intercept.– Example: find the y-intercept of 4x – 3y = 6

4(0) – 3y = 6 substitute 0 for x y = -2, so the graph crosses the y-axis at the point (0, -2)

• The point on the graph where the line crosses the x-axis is called the x-intercept.– Example: find the x-intercept of 3x + 5y = 9

3x + 5(0) = 9 substitute 0 for y x = 3, so the graph crosses the x-axis at the point (3, 0)

2-2 Practice

1. Determine if the following are linear equations. If so, write the equation in standard form and determine A, B, and C.

a. 4x + 3y = 10 c. 5 – 3y = 8x

b. x2 + y = 2 d. 1/x + 4y = -5

2. Find the x- and y-intercepts of the following:a. 4x – 3y = -12

b. ½ y + 2 = ½ x

Answers: 1)a) yes; A = 4, B = 3, C = 10 b) no c) yes; A = 8, B = 3, C = 5 d) no

2)a) x-intercept: -3 y-intercept: 4 b) x-intercept: 4 y-intercept: -4

2-3 Slope• The slope of a line is the change in y over

the change in x.• If a line passes through the points (x1, y1)

and (x2, y2), then the slope is given by

m = y2 – y1 , where x1 ≠ x2.

x2 – x1

• In an equation with the from y = mx + b, m is the slope and b is the y-intercept.

• Two lines with the same slope are parallel.• If the product of the slopes of two lines is -1,

then the lines are perpendicular.

2-3 Practice1. Find the slope of the following:

a. (-2, 4) (3, -6) d. (-1, 8) (14, 8)

b. (3.5, -2) (0, -16) e. 12x + 3y – 6 = 0

c. y = 3x + b f. y = -7

2. Determine whether the following lines are perpendicular or parallel by finding the slope.

a. (4, -2) (6, 0), (7, 3) (6, 2)

b. y = 2x – 3, (6, 6) (4, 7)

Answers: 1)a) -2 b) 4 c) 3 d) 0 e) -4 f) 0 2)a) 1; parallel b) -1; perpendicular

2-4 Writing Linear Equations

• The form y = mx + b is called slope-intercept form, where m is the slope and b is the y-intercept.

• The point-slope form of the equation of a line is y – y1 = m(x – x1). Here (x1, y1) are the coordinates of any point found on that line.

2-4 Writing Linear Equations

• Example: Find the slope-intercept form of the equation passing through the point (-3, 5) with a slope of 2.

y = mx + b

5 = (2)(-3) + b

5 = -6 + b

b = 11

y = 2x + 11

2-4 Writing Linear Equations

• Example: Find the point-slope form of the equation of a line that passes through the points (1, -5) and (0, 4).

m = y2 – y1 y – y1 = m(x – x1)

x2 – x1 y – (-5) = (-1)(x – 1)

m = 4 + 5 y + 5 = -x + 1

0 – 1 y = -x – 4

m = 9

-1

m = -1

2-4 Practice

1. Find the slope-intercept form of the following:a. a line passing through the point (0, 5) with a slope

of -7

b. a line passing through the points (-2, 4) and (3, 14)

2. Find the point-slope form of the following:a. a line passing through the point (-2, 6) with a slope

of 3

b. a line passing through the points (0, -9) and (-2, 1)

Answers: 1)a) y = -7x + 5 b) y = 2x + 8 2)a) y = 3x + 12 b) y = -5x -9

2-5 Modeling Real-World Data Using Scatter Plots

• Plotting points that do not form a straight line forms a scatter plot.

• The line that best represents the points is the best-fit line.

• A prediction equation uses points on the scatter plot to approximate through calculation the equation of the best-fit line.

2-5 Practice

1. Plot the following data. Approximate the best-fit line by creating a prediction equation.

Person ACT Score

1 15

2 19

3 21

4 28

5 30

6 35

Answers: 1) y = 4x + 11

2-6 Special Functions

• Whenever a linear function has the form y = mx + b and b = 0 and m ≠ 0, it is called a direction variation.

• A constant function is a linear function in the form y = mx + b where m = 0.

• An identity function is a linear function in the form y = mx + b where m = 1 and b = 0.

2-6 Special Functions

• Step functions are functions depicted in graphs with open circles which mean that the particular point is not included.– Example:

2-6 Special Functions• A type of step function is the greatest integer

function which is symbolized as [x] and means “the greatest integer not greater than x.”– Examples: [8.2] = 8 [3.9] = 3

[5.0] = 5 [7.6] = 7

• An absolute value function is the graph of the function that represents an absolute value.– Examples: |-4| = 4 |-9| = 9

2-6 Practice1. Identify each of the following as constant,

identity, direct variation, absolute value, or greatest integer function

a. h(x) = [x – 6] e. f(x) = 3|-x + 1|

b. f(x) = -½ x f. g(x) = x

c. g(x) = |2x| g. h(x) = [2 + 5x]

d. h(x) = 7 h. f(x) = 9x

2. Graph the equation y = |x – 6|

2-6 Answers

• Answers: 1)a) greatest integer function b) direct variation c) absolute value d) constant e) absolute value f) identity g) greatest integer function h) direct variation 2)

2-7 Linear Inequalities• Example: Graph 2y – 8x ≥ 4

– Graph the “equals” part of the equation. 2y – 8x = 4

2y = 8x + 4

y = 4x + 2

x-intercept

0 = 4x + 2

-2 = 4x

-1/2 = x

y-intercept

y = 4(0) +2

y = 2

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

2-7 Linear Inequalities– Use “test points” to determine which side of the line

should be shaded. (2y – 8x ≥ 4)

(-2, 2)

2(2) – 8(-2) ≥ 4

4 – (-16) ≥ 4

20 ≥ 4 true

(0, 0)

2(0) – 8(0) ≥ 4

0 – 0 ≥ 4

0 ≥ 4 false

– So we shade the side of the line that includes the “true” point, (-2, 2)

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

2-7 Linear Inequalities• Example: Graph 12 < -3y – 9x

– Graph the line.12 ≠ -3y – 9x

3y ≠ -9x – 12

y ≠ -3x – 4

x-intercept

0 = -3x – 4

4 = -3x

-4/3 = x

y-intercept

y = 3(0) – 4

y = -4

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

2-7 Linear Inequalities– Use “test points” to determine which side of the line

should be shaded. (12 < -3y – 9x) (-3, -3)

12 < -3(-3) – 9(-3)

12 < 9 + 27

12 < 36 true

(0, 0)

12 < -3(0) – 9(0)

12 < 0 – 0

12 < 0 false

– So we shade the side of the line that includes the “true” point, (-3, -3)

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

2-7 Problems

1. Graph each inequality.a. 2x > y – 4 e. 2y ≥ 6|x|

b. 5 ≥ y f. 42x > 7y

c. 4 < -2y g. |x| < y + 2

d. y ≤ |x| + 3 h. x – 4 ≤ 8y

2-7 Answers1)a) b)

c) d)

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

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10

2-7 Answers1)e) f)

g) h)

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10

-10 -8 -6 -4 -2 2 4 6 8 10

-10

-8

-6

-4

-2

2

4

6

8

10