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Lesson 4-1/4-2
Triangles
Isosceles &
Fundamentals
Lesson 3-2: Isosceles Triangle
1
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
2
Isosceles Triangle Theorems
If two sides of a triangle are congruent, then the angles
____________________ 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,
50
_____ + _____ + ______ = 180
x
Lesson 3-2: Isosceles Triangle
3
Isosceles Triangle Theorems
If two sides of a triangle are congruent, then the angles
____________________ 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,
___________ = __________
2x-3
50
Lesson 3-2: Isosceles Triangle
4
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
_________ = ____________
B
50 
50 
C
Lesson 3-2: Isosceles Triangle
5
Isosceles Triangle Theorems
The sum of the sides will never equal _______________
Sides can only be ____________________________ 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
B
50 
50 
C
x+15
Lesson 3-2: Isosceles Triangle
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