* 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. Obj. 18 Isosceles and Equilateral
The student is able to (I can):
• 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

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
Converse of
the Isosceles
Triangle
Theorem
If two sides of a triangle are congruent,
then the angles opposite the sides are
congruent.
C
B
A
AB @ CBÐA @ ÐC
If two angles of a triangle are congruent,
then the sides opposite those angles are
congruent.
F
E
D
ÐD @ ÐFDE @ FE

4. Equilateral
Triangle
Corollary
Converse of
the Equilateral
Triangle
Corollary
If a triangle is equilateral, then it is
equiangular.
C
B
A
@ @
AB BC CA
Ð @ Ð @ Ð
A B C
If a triangle is equiangular, then it is
equilateral.
Ð @ Ð @ Ð
D E F
DE EF FD
@ @
F
E
D

5. Practice
1. mÐS
2. mÐK
3. mÐS
35°
S
K
Y
S
E
A
22°

6. Practice
1. mÐS
2. mÐK
180 — (35 + 35)
180 — 70
110°
3. mÐS
180 — 22 = 158
35°
S
K
Y
S
E
A
22°
= 35°
35°
110°
= °
158
79
2
79°