1. Lesson 7-2
Similar Polygons

2. Similar Polygons-polygons with the same shape, but not necessarily the same size.
similar sign (∼)

3. Example: Write a similar statement for the polygons
ABCD ∼ EFGH
Be careful how you write them. They have
to be the similar parts.

4. Scale Factor:  The ratio of the lengths of the corresponding sides of two similar polygons. Similarity Ratio:  Another name for scale factor.
Example:
a) 𝑥 10 = 9 6   
So using cross products, we can say that 6x = 90,
so x = 15
15 10 = 12 3𝑦−1
So, 120 = 15(3y – 1)  
using cross products, or 120 = 45y – 15,
so 135 = 45y
Therefore y = =3

5. Since the lengths of the sides of similar figures can be multiplied by the scale factor, so can the perimeters of the similar figures.

6. Example: 8 𝑥 = 4 3 Since AB∼QR and CD∼RS, we can write a proportion.
8 𝑥 = 4 3
4x = 24
X = 6
So QR = 6
So the sides of ABCDE are 8, 8, 4, 6, and 4, which has a perimeter of 30 units.
Since we know the scale factor is , we can say that the perimeter of RQPTS is 22.5 units.