Identify isosceles and equilateral triangles by side length and angle measure
Use the Isosceles Triangle Theorem to solve problems
Use the Equilateral Triangle Corollary to solve problems
1. Isosceles and Equilateral Triangles
The student is able to (I can):
• Identify isosceles and equilateral triangles by side length
and angle measureand angle measure
• Use the Isosceles Triangle Theorem to solve problems
• Use the Equilateral Triangle Corollary to solve problems
2. Parts of an Isosceles Triangle:
1
legs
vertex angle
2 3
base
base angles
Note: the base is the side opposite the
vertex angle, not necessarily the side on
the “bottom”.
3. Isosceles
Triangle
Theorem
If two sides of a triangle are congruent,
then the angles opposite the sides are
congruent.
C
B
A
AB CB A C≅ ⇒ ∠ ≅ ∠
Converse of
the Isosceles
Triangle
Theorem
If two angles of a triangle are congruent,
then the sides opposite those angles are
congruent.
F
E
D
D F DE FE∠ ≅ ∠ ⇒ ≅
4. Equilateral
Triangle
Corollary
Converse of
If a triangle is equilateral, then it is
equiangular.
If a triangle is equiangular, then it is
C
B
A
AB BC CA
A B C
≅ ≅
⇒ ∠ ≅ ∠ ≅ ∠
Converse of
the Equilateral
Triangle
Corollary
If a triangle is equiangular, then it is
equilateral.
D E F
DE EF FD
∠ ≅ ∠ ≅ ∠
⇒ ≅ ≅
F
E
D
