Finding the 3rd Angle in a Triangle

If you add all three interior angle measures together in a triangle it will always equal 180°.. To find a third angle you will subtract the sum of the two given angles from 180°. Look at the 3 examples below.

72° + 50° + 58° = 180°

103° + 47° + 30° = 180°

90° + 36° + 54° = 180°

Now let's look at what to do when we are given 2 angles but we are missing the 3rd angle.

Step 1: add the two angles that you are given together
Step 2: subtract your answer from 180


Example 1:

Step 1: 30° + 35° = 65° Step 2: 180° - 65° = 115° m B = 115°

Example 2:

Step 1: 90° + 20° = 110° Step 2: 180° - 110° = 70° m F = 70°

Example 3:

Step 1: 125° + 20° = 145° Step 2: 180° - 145° = 35° x° = 35

The key to finding the missing angle measure of a triangle is to remember that the sum of the interior angles of any triangle is always 180°. If you know 2 angles then you can subtract their sum from 180° to find the measure of the 3rd angle.

Related Links:
Math
Geometry
Topics
Classifying Triangles by Angles


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