1. G9 - Congruence Postulates for Triangles
Algebra 1B

2. A well known Triangle Theorem
The sum of the interior angles in a triangle is 180˚.
Triangle Sum Theorem:
We will leave the proof until later

3. Exterior Angle of a Triangle
3 is called an exterior angle of the triangle
An exterior angle is created by lengthening one of the sides of the triangle.
An exterior angle forms a linear pair with one of the angles of the triangle.
The other two angles of the triangle are called the remote interior angles.

4. Exterior Angle formal Defintion
If C is between A and D, then BCD is an exterior angle of ABC

5. Every triangle has 6 exterior angles.2 1 3 X 4 6 5 X
X = not exterior angles

6. Exterior Angle Theorem
The measure of the exterior angle in a triangle is equal to the sum of the measures of the remote interior angles.

7. Congruent Triangles  ABC  DEF
If all six pairs of corresponding parts (sides and angles) are
congruent, then the triangles are congruent.
Sides are
congruent
Angles are
congruent
Triangles are
congruent If
and
then
 ABC  DEF
1. AB DE
A D
2. BC EF
B E
3. AC DF
C F

9. What about Angle- Side – Side?

10. Isosceles Triangle
An Isosceles Triangle has two congruent sides called the legs.
The angle formed by the two legs is the vertex angle
The third side is called the base.
The two angles adjacent to the base are called the base angles.
Triangle Fundamentals

11. Isosceles Triangle Theorem:
Triangle Fundamentals

12. Converse of Isosceles Triangle Theorem:
Triangle Fundamentals