Warm-Up• Find the value of x.

x + 25

3x - 3

GEOMETRY 4-8Isosceles and Equilateral Triangles

Parts of an Isosceles Triangle

Base: the side opposite the vertex angle

Base angle Base angle

Leg: a congruent side Leg: a congruent side

Vertex angle: formed by the two legs

Theorem

• Isosceles Triangle Theorem:• If two sides of a triangle are congruent, then the angles opposite those sides are congruent.

If

Then

Examples• Find the value of x.

55°

x + 4°1)

2)

Equilateral Triangles

• Equilateral Theorem:• All equilateral triangles are equiangular.• All equiangular triangles are equilateral.

Examples• Find the value of x. Find the value of y.