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7.2 Similar Polygons
Textbook page 365
Similar Polygons

Two polygons are similar if corresponding
angles are congruent and corresponding
sides are proportional
A
B
W
D
X
Z
C
Y
ABCD ~ WXYZ if:
A  W
C  Y
B  X
big trapezoid
AB BC CD DA



WX XY YZ ZW
little trapezoid
D  Z
Are the triangles similar?
25
15
6
20




8
All of the corresponding angles are congruent.
Are the corresponding sides proportional?
big triangle
little triangle

10
15 25 20
?


6 10 8
2.5 = 2.5 = 2.5
Yes, they are similar
What is the scale factor of A:B?
25
15
A
6
20
big triangle (A)
little triangle (B)


10
B
8
15 25 20


6 10 8
Reduce the fractions (they are all the same if the
triangles are similar)
The scale factor is 5
2
A
W
10
E
20
D
B
4
V
8
15
Z
C
X
n
Y
ABCDE ~ WXYZV, solve for n
big pentagon
little pentagon
=
n
n=6
Perimeters of Similar Polygons
Theorem:

If two polygons are similar, then the ratios
of their perimeters is equal to the ratios of
their corresponding side lengths.
15
5
A
Find the ratio of the perimeter of A
to the perimeter of B
9
B
15 9
or
5
3
reduces to
3
3
1
Examples on pg 366-368
1)
 2)
 3)
 4)

yes, XYZ ~ DEF, 3/2
no
18
½
Homework
Assignment # 4
 Pg 368
 Problems 1-20, 30-37
