1. A C B Triangle Sum Theorem (4.1)
The sum of the interior angles of a 
triangle is 180o A B C

2. Find the
Find a Missing Angle of a Triangle
ex#1 A B C

3. Find the A B C
Find a Missing Angle of a Triangle
ex#2

4. The acute angles of a right triangle are complimentary.
Corollary to the Triangle Sum Theorem
The acute angles of a right 
triangle are complimentary. In
, if
, then... A B C

5. Question: What is a corollary?
A corollary to a theorem is a 
statement that can be proven 
easily from the theorem.

6. ex#3 D B A A B C D

7. A B C

8. You Try!!

9. When the sides of a triangle are extended, other angles are formed
When the sides of a triangle are extended, 
other angles are formed.The three original 
angles are the ____________.
interior angles
interior angles

10. The angles that are adjacent to the 
interior angles are the _____________.
exterior angles
exterior angle
interior angles
exterior angle
exterior angle

11. Find the
ex#4

12. Find the
ex#5

13. Find the
ex#6

14. Exterior Angle Theorem
The measure of an exterior angle of a 
triangle is the sum of the measures of the 
two non-adjacent, interior angles. A B C 1

15. Find the
ex#7

16. HOMEWORK:
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