Triangle Sum Theorem

The sum of the angles in a triangle is equal to a straight angle (180 degrees).

We can use the fact that the sum of the angles in a triangle is 180 degrees to solve Geometry problems.

Example Problem

We know the measure of two angles in a triangle.

1 = 25

and

3 = 75

Find the number of degrees in

2 Step 1: Set up the equation

1 +

2 +

3 = 180

Step 2: Rewrite the equation using the information given in the problem: 25 +

2 + 75 = 180 Step 3: Add the numbers that you can on the left of the equal sign 100 +

2 = 180 Step 4: Subtract 100 from both sides 2 = 180