Download Lesson 3-2 Isosceles Triangles

Lesson 3-2
Isosceles
Triangles
Lesson 3-2: Isosceles Triangle
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Parts of an Isosceles Triangle


An isosceles triangle is a triangle with two congruent
sides.
The congruent sides are called legs and the third side is
called the base.
3
Leg
Leg
1 and2 are base angles
3 is the vertex angle
1
2
Base
Lesson 3-2: Isosceles Triangle
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Isosceles Triangle Theorems
If two sides of a triangle are congruent, then the angles opposite
those sides are congruent.
A
If AB  AC , then B  C.
B
C
Example: Find the value of x. By the Isosceles Triangle Theorem,
the third angle must also be x.
Therefore, x + x + 50 = 180
50
2x + 50 = 180
2x = 130
x
x = 65
Lesson 3-2: Isosceles Triangle
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Isosceles Triangle Theorems
If two angles of a triangle are congruent, then the sides opposite
those angles are congruent.
A
If B  C , then AB  AC.
B
C
Example: Find the value of x. Since two angles are congruent, the
A
sides opposite these angles must be
congruent.
3x - 7
x+15
3x – 7 = x + 15
2x = 22
50  C
B 50 
X = 11
Lesson 3-2: Isosceles Triangle
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