proportions - mr. gilliam's classroom · triangles in the real world do you know how tall your...

Proportions

Wednesday, May 4, 2011

ProportionsWhat are proportions?


ProportionsWhat are proportions? - If two ratios are equal, they form a proportion.

Proportions can be used in geometry when working with similar figures. 1

248

= 1:3 = 3:9



Proportions can be used in geometry when working with similar figures.

What do we mean by similar?

12

48

= 1:3 = 3:9



Proportions can be used in geometry when working with similar figures.

What do we mean by similar? - Similar describes things which have the same

shape but are not the same size.

12

48

= 1:3 = 3:9


ExamplesThese two stick figures are similar. As you can see both are the same shape. However, the bigger stick figure’s dimensions are exactly twice the smaller.

So the ratio of the smaller figure to the larger figure is 1:2 (said “one to two”). This can also be written as a fraction of ½.

A proportion can be made relating the height and the width of the smaller figure to the larger figure:

2 feet

4 feet

8 feet

4 feet4 ft2 ft

=8 ft4 ft


Solving Proportional Problems

So how do we use proportions and similar figures?

Using the previous example we can show how to solve for an unknown dimension.

2 feet

4 feet

8 feet

? feet


Solving Proportion ProblemsFirst, designate the unknown side as x. Then, set up an equation using proportions. What does the numerator represent? What does the denominator represent?

Then solve for x by cross multiplying: 2 feet

4 feet

8 feet

? feet

4 ft

2 ft=

8 ft

x ft

4x = 16

X = 4


Try One Yourself

4 feet

8 feet 12 feet

x feet


Try One Yourself

Knowing these two stick figures are similar to each other, what is the ratio between the smaller figure to the larger figure?

4 feet

8 feet 12 feet

x feet


Try One Yourself

Knowing these two stick figures are similar to each other, what is the ratio between the smaller figure to the larger figure?

Set up a proportion. What is the width of the larger stick figure?

4 feet

8 feet 12 feet

x feet


Similar ShapesIn geometry similar shapes are very important. This is because if we know the dimensions of one shape and one of the dimensions of another shape similar to it, we can figure out the unknown dimensions.


Triangle and Angle ReviewToday we will be working with right triangles. Recall that one of the angles in a right triangle equals 90o. This angle is represented by a square in the corner.

To designate equal angles we will use the same symbol for both angles.

90o angle

equal angles


Proportions and TrianglesWhat are the unknown values on these triangles?

16 m

20 m

4 m

3 m

x m

y m

First, write proportions relating the two triangles.



16 m

20 m

4 m

3 m

x m

y m


4 m

16 m=

3 m

x m

4 m

16 m=

y m

20 m



16 m

20 m

4 m

3 m

x m

y m


4 m

16 m=

3 m

x m

4 m

16 m=

y m

20 m

Solve for the unknown by cross multiplying.



16 m

20 m

4 m

3 m

x m

y m


4 m

16 m=

3 m

x m

4 m

16 m=

y m

20 m

Solve for the unknown by cross multiplying.

4x = 48

x = 12

16y = 80

y = 5


Triangles in the Real WorldDo you know how tall your school building is?There is an easy way to find out using right triangles.

To do this create two similar triangles using the building, its shadow, a smaller object with a known height (like a yardstick), and its shadow.

The two shadows can be measured, and you know the height of the yard stick. So you can set up similar triangles and solve for the height of the building.


Solving for the Building’s HeightHere is a sample calculation for the height of a building:

48 feet

4 feet

3 feet

yardstick

building

x feet



48 feet

4 feet

3 feet

yardstick

building

x feetx ft3 ft

=48 ft4 ft



48 feet

4 feet

3 feet

yardstick

building

x feetx ft3 ft

=48 ft4 ft

4x = 144

x = 36



48 feet

4 feet

3 feet

yardstick

building

x feetx ft3 ft

=48 ft4 ft

4x = 144

x = 36

The height of the building is 36 feet.


Accuracy and ErrorDo you think using proportions to calculate the height of the building is better or worse than actually measuring the height of the building?

Determine your height by the same technique used to determine the height of the building. Now measure your actual height and compare your answers.

Were they the same? Why would there be a difference?


proportions - mr. gilliam's classroom · triangles in the real world do you know how tall your...

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