1. 4.6: Isosceles and Equilateral Triangles
Geometry
4.6:
Isosceles and Equilateral Triangles

2. The angle opposite the base is called the vertex angle.
The two angles in an isosceles triangle adjacent to the base of the triangle are called base angles.
The angle opposite the base is called the vertex angle.
Vertex Angle
Base Angle
Base Angle

3. Base Angles Theorem
If two sides of a triangle are congruent, then the angles opposite them are congruent. A C B

4. Converse to the Base Angles Theorem
If two angles of a triangle are congruent, then the sides opposite them are congruent.

5. Corollary to the Base Angles Theorem
If a triangle is equilateral, then it is equiangular.

6. Corollary to the Converse of the Base Angles Theorem
If a triangle is equiangular, then it is equilateral.

7. C A C B A B A B C
Yes
Yes No

8. Hypotenuse-Leg (HL) Congruence Theorem
If the hypotenuse and a leg of a right triangle are congruent to the hypotenuse and a leg of a second right triangle, then the two triangles are congruent. A D B F C E

9. Practice Problems
Find the measure of the missing angles and tell which theorems you used. B B A C
50° A C
m C=50°
(Base Angle Theorem)
m B=80°
(Triangle Sum Theorem)
m A=60°
m B=60°
m C=60°
(Corollary to the Base Angles Theorem)

10. More Practice Problems
Is there enough information to prove the triangles are congruent? S T U R V No
Yes No W

11. Factoring Review
Solve 𝑥 2 +𝑥−12

12. Homework
Page 287: 9-22 all, all, 38-39