1. Preview
Warm Up
California Standards
Lesson Presentation

2. Warm Up Solve each proportion. = = 1. b = 10 2. y = 8 = = 3. 4. m = 5230 3 9 = 56 35 y 5 = 1.
b = 10 2.
y = 8 4 12 p 9 = 56 m 28 26 = 3. 4.
m = 52
p = 3

3. Preparation for MG1.2 Construct and read drawings and models made to scale.
California
Standards

4. Vocabulary
similar
corresponding sides
corresponding angles

5. Similar figures have the same shape, but not necessarily the same size.
Corresponding sides of two figures are in the same relative position, and corresponding angles are in the same relative position. Two figures are similar if the lengths of corresponding sides are proportional and the corresponding angles have equal measures.

6. 43°

7. A is read as “angle A. ” ∆ABC is read as “triangle ABC
A is read as “angle A.” ∆ABC is read as “triangle ABC.” “∆ABC ~∆EFG” is read as “triangle ABC is similar to triangle EFG.”
Reading Math

8. Additional Example 1: Identifying Similar Figures
Which rectangles are similar?
Since the three figures are all rectangles, all the angles are right angles. So the corresponding angles are congruent.

9. Additional Example 1 Continued
Compare the ratios of corresponding sides to see if they are equal.
length of rectangle J
length of rectangle K
width of rectangle J
width of rectangle K 10 5 4 2 ? =
20 = 20
The ratios are equal. Rectangle J is similar to rectangle K. The notation J ~ K shows similarity.
length of rectangle J
length of rectangle L
width of rectangle J
width of rectangle L 10 12 4 5 ? =
50  48
The ratios are not equal. Rectangle J is not similar to rectangle L. Therefore, rectangle K is not similar to rectangle L.

10. Which rectangles are similar?
Check It Out! Example 1
Which rectangles are similar?
8 ft A
6 ft B C
5 ft
4 ft
3 ft
2 ft
Since the three figures are all rectangles, all the angles are right angles. So the corresponding angles are congruent.

11. Check It Out! Example 1 Continued
Compare the ratios of corresponding sides to see if they are equal.
length of rectangle A
length of rectangle B
width of rectangle A
width of rectangle B 8 6 4 3 ? =
24 = 24
The ratios are equal. Rectangle A is similar to rectangle B. The notation A ~ B shows similarity.
length of rectangle A
length of rectangle C
width of rectangle A
width of rectangle C 8 5 4 2 ? =
16  20
The ratios are not equal. Rectangle A is not similar to rectangle C. Therefore, rectangle B is not similar to rectangle C.

12. Additional Example 2: Finding Missing Measures in Similar Figures
A picture 10 in. tall and 14 in. wide is to be scaled to 1.5 in. tall to be displayed on a Web page. How wide should the picture be on the Web page for the two pictures to be similar?
Set up a proportion. Let w be the width of the picture on the Web page.
width of a picture
width on Web page
height of picture
height on Web page 14 w = 10
1.5
14 ∙ 1.5 = w ∙ 10
Find the cross products.
21 = 10w
10w 10 21 =
Divide both sides by 10.
2.1 = w
The picture on the Web page should be 2.1 in. wide.

13. Check It Out! Example 2
A painting 40 in. long and 56 in. wide is to be scaled to 10 in. long to be displayed on a poster. How wide should the painting be on the poster for the two pictures to be similar?
Set up a proportion. Let w be the width of the painting on the Poster.
width of a painting
width of poster
length of painting
length of poster 56 w = 40 10
56 ∙ 10 = w ∙ 40
Find the cross products.
560 = 40w
40w 40
560 =
Divide both sides by 40.
14 = w
The painting displayed on the poster should be 14 in. long.

14. Additional Example 3: Business Application
A T-shirt design includes an isosceles triangle with side lengths 4.5 in, 4.5 in., and 6 in. An advertisement shows an enlarged version of the triangle with two sides that are each 3 ft. long. What is the length of the third side of the triangle in the advertisement?
side of small triangle base of small triangle
side of large triangle base of large triangle =
3 ft
x ft
4.5 in.
6 in. =
Set up a proportion.
4.5 • x = 3 • 6
Find the cross products.
4.5x = 18
Multiply.

15. Additional Example 3 Continued
A T-shirt design includes an isosceles triangle with side lengths 4.5 in, 4.5 in., and 6 in. An advertisement shows an enlarged version of the triangle with two sides that are each 3 ft. long. What is the length of the third side of the triangle in the advertisement?
x = = 4 18
4.5
Solve for x.
The third side of the triangle is 4 ft long.

16. Check It Out! Example 3
A flag in the shape of an isosceles triangle with side lengths 18 ft, 18 ft, and 24 ft is hanging on a pole outside a campground. A camp t-shirt shows a smaller version of the triangle with two sides that are each 4 in. long. What is the length of the third side of the triangle on the t-shirt?
side of large triangle side of small triangle
base of large triangle base of small triangle =
24 ft
x in.
18 ft
4 in. =
Set up a proportion.
18 ft • x in. = 24 ft • 4 in.
Find the cross products.
18x = 96
Multiply.

17. Check It Out! Example 3 Continued
A flag in the shape of an isosceles triangle with side lengths 18 ft, 18 ft, and 24 ft is hanging on a pole outside a campground. A camp t-shirt shows a smaller version of the triangle with two sides that are each 4 in. long. What is the length of the third side of the triangle on the t-shirt?
x =  5.3 96 18
Solve for x.
The third side of the triangle is about 5.3 in. long.

18. Lesson Quiz
Use the properties of similar figures to answer each question.
1. Which rectangles are similar?
A and B are similar.
2. Karen enlarged a 3 in. wide by 5 in. tall photo into a poster. If the poster is 2.25 ft wide, how tall is it?
3.75 ft
3. A rectangular house is 32 ft wide and 68 ft long. On a blueprint, the width is 8 in. Find the length on the blueprint.
17 in.