1. Triangles Isosceles & Fundamentals
Lesson 4-1/4-2
Triangles
Isosceles & Fundamentals
Lesson 3-2: Isosceles Triangle

2. 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
Base 2 1 Ð
1 and
2 are base angles
3 is the vertex angle
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 B C
Example:
Find the value of x.
By the Isosceles Triangle Theorem,
the third angle must also be x.
Therefore, _____ + _____ + ______ = 180 x 50
Lesson 3-2: Isosceles Triangle

4. Isosceles Triangle Theorems
If two sides of a triangle are congruent, then the angles ____________________ those sides are congruent. A 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

5. Isosceles Triangle Theorems
If two angles of a triangle are congruent, then the sides opposite those angles are congruent. A B C
Example:
Find the value of x.
Since two angles are congruent, the
sides opposite these angles must be
congruent.
_________ = ____________ 50
3x - 7
x+15 A B C
Lesson 3-2: Isosceles Triangle

6. Isosceles Triangle Theorems
The sum of the sides will never equal _______________
Sides can only be ____________________________ A B C
Example:
Find the value of x.
Since two angles are congruent, the
sides opposite these angles must be
congruent. 50
3x - 7
x+15 A B C
Lesson 3-2: Isosceles Triangle