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Proportions. 1. 4. =. 1:3 = 3:9. 2. 8. Proportions. What are proportions? - If two ratios are equal, they form a proportion. Proportions can be used in geometry when working with similar figures. What do we mean by similar?

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proportions2

1

4

=

1:3 = 3:9

2

8

Proportions

What are proportions?

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

examples

8 ft

4 ft

=

2 ft

4 ft

Examples

These 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:

8 feet

4 feet

2 feet

4 feet

solving proportional problems
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.

8 feet

4 feet

2 feet

? feet

solving proportion problems

8 ft

4 ft

=

2 ft

x ft

Solving Proportion Problems

First, 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:

8 feet

4 feet

2 feet

4x = 16

X = 4

? feet

try one yourself
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?

8 feet

12 feet

4 feet

x feet

similar shapes
Similar Shapes

In 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 review
Triangle and Angle Review

Today 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 triangles

y m

3 m

4 m

4 m

=

=

16 m

16 m

20 m

x m

Proportions and Triangles

What are the unknown values on these triangles?

First, write proportions relating the two triangles.

20 m

16 m

Solve for the unknown by cross multiplying.

x m

4x = 48

x = 12

16y = 80

y = 5

y m

4 m

3 m

triangles in the real world
Triangles in the Real World

Do 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 height

48 ft

x ft

=

3 ft

4 ft

Solving for the Building’s Height

Here is a sample calculation for the height of a building:

building

x feet

48 feet

4x = 144

x = 36

yardstick

3 feet

The height of the building is 36 feet.

4 feet

accuracy and error
Accuracy and Error

Do 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?