What if you wanted to create a scale drawing using scale factors? This technique takes a small object, like the handprint below, divides it up into smaller squares and then blows up the individual squares. Either trace your hand or stamp it on a piece of paper. Then, divide your hand into 9 squares, like the one to the right, probably

.

### Area and Perimeter of Similar Polygons

Polygons are similar when the corresponding angles are equal and the corresponding sides are in the same proportion. The scale factor for the sides of two similar polygons is the same as the ratio of the perimeters. In fact, the ratio of any part of two similar shapes (diagonals, medians, midsegments, altitudes, etc.) is the same as the scale factor. The ratio of the areas is the ** square** of the scale factor. An easy way to remember this is to think about the units of area, which are always

**Therefore, you would always**

*squared.***the scale factor to get the ratio of the areas.**

*square***Area of Similar Polygons Theorem:** If the scale factor of the sides of two similar polygons is

#### Finding the Scale Factor and Ratio of Perimeters

The two rectangles below are similar. Find the scale factor and the ratio of the perimeters.

The scale factor is

#### Calculating Area

Find the area of each rectangle from the previous Example. Then, find the ratio of the areas.

The ratio of the areas would be

The ratio of the sides, or scale factor was

#### Finding the Ratio of Areas

Find the ratio of the areas of the rhombi below. The rhombi are similar.

There are two ways to approach this problem. One way would be to use the Pythagorean Theorem to find the length of the

#### Handprint Problem Revisited

You should end up with an

### Examples

#### Example 1

Two trapezoids are similar. If the scale factor is

First, the ratio of the areas would be

#### Example 2

Two triangles are similar. The ratio of the areas is

The scale factor is

#### Example 3

Using the ratios from #2, find the length of the base of the smaller triangle if the length of the base of the larger triangle is 24 units.

All you would need to do is multiply the scale factor we found in #2 by 24.

### Review

Determine the ratio of the areas, given the ratio of the sides of a polygon.

35 14 72 611

Determine the ratio of the sides of a polygon, given the ratio of the areas.

136 481 499 25144

This is an equilateral triangle made up of 4 congruent equilateral triangles.

- What is the ratio of the areas of the large triangle to one of the small triangles?
- What is the scale factor of large to small triangle?
- If the area of the large triangle is
20 units2 , what is the area of a small triangle? - If the length of the altitude of a small triangle is
23√ , find the perimeter of the large triangle. - Carol drew two equilateral triangles. Each side of one triangle is 2.5 times as long as a side of the other triangle. The perimeter of the smaller triangle is 40 cm. What is the perimeter of the larger triangle?
- If the area of the smaller triangle is
75 cm2 , what is the area of the larger triangle from #13? - Two rectangles are similar with a scale factor of
47 . If the area of the larger rectangle is294 in2 , find the area of the smaller rectangle. - Two triangles are similar with a scale factor of
13 . If the area of the smaller triangle is22 ft2 , find the area of the larger triangle. - The ratio of the areas of two similar squares is
1681 . If the length of a side of the smaller square is 24 units, find the length of a side in the larger square. - The ratio of the areas of two right triangles is
23 . If the length of the hypotenuse of the larger triangle is 48 units, find the length of the smaller triangle’s hypotenuse.

Questions 19-22 build off of each other. You may assume the problems are connected.

- Two similar rhombi have areas of
72 units2 and162 units2 . Find the ratio of the areas. - Find the scale factor.
- The diagonals in these rhombi are congruent. Find the length of the diagonals and the sides.
- What type of rhombi are these quadrilaterals?
- The area of one square on a game board is exactly twice the area of another square. Each side of the larger square is 50 mm long. How long is each side of the smaller square?

### Review (Answers)

To view the Review answers, open this PDF file and look for section 10.7.