Angles of Triangles

Every triangle has 3 angles.  The prefix tri means three, that's why they named it a TRIANGLE. 


All of the angles of any triangle always add up to 180 degrees total.  Look at these examples: 

Sometimes, when we are given the degree measures of the angles, we are only given the measure of two out of the three angles.  You may be asked to find the measure of the missing angle.  Look at the step below. 

Steps to Find the Missing Angle

1.  Add up the measure of the two given sides. 
2.  Subtract the sum from 180.
3.  The difference is the measure of the third angle. 

Example: 

1.     50 + 38 = 88

2.     180 - 88 =  92

3.    ? = 92


The measure of the missng angle is 92°






Sometimes you may not be given a picture, but just the measures of two of the angles and asked to find the third angle.  Look at the example below: 

Two of the measures of the angles of a triangle are 55° and 60°.  What is the measure of the third angle? 

1.     55 + 65 = 120

2.     180 - 120 =  40

3.    Third angle = 40°



Take quiz 1  and quiz 2 to practice finding the missing angle. 
(skip question 3 on quiz 2)

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