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Congruence and Similarity
Similar• Two shapes are similar if
their corresponding angles are equal and their corresponding sides are in proportion.
Top Length 5 cm
Bottom Length 10 cm Top Length 3 cm
Bottom Length 6 cm
600 600
1200 1200
Congruent • Two shapes are congruent if
all corresponding angles AND sides are equal. • If two shapes are congruent,
they are also similar.
Top Length 4 cm
110 0 110 0
70 0 70 0
Bottom Length 5 cm
Perimeter• To find the perimeter of
any polygon, add up the length of all sides.
Triangles• Triangles have 3 angles that
add up to 180 degrees.• To find the area of a
triangle, multiply ½ the base x the height.
Rectangles•Rectangles have 4 right
angles that add up to 360 degrees.• To find the area of a
rectangle, multiply the length x the width.
Scale Factor• The ratio of corresponding
side lengths of two similar figures.•All congruent shapes have
a scale factor of 1:1.
ALength = 10 cm
Width = 5 cm
Length = 4 cm
Width = 2 cm
The scale factor from A to B is 5:2.The ratio of areas from A to B is 25:4
B
Missing Side• You can find the missing
side length on one of two similar figures by using a proportion involving scale factor.
ALength = 20 cm
Width = 5 cm
Length = 4 cm
Width = ? cm
Set up a proportion. Cross multiply and divide.20 = 54 xSo, x = 1 cm B
A and B are similar.
Missing Angle• You can find the measure
of a missing angle on one of two similar triangles by subtracting from 180 degrees.
500
700
y z
x
d
e f
If triangles xyz and def are similar, what is the measure of angle y?
Y = 60 degrees because 50 + 70 = 120.180 – 120 = 60.