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How to Find Outside Angles in Triangles

A triangle not only has inside angles but outside angles as well. When a triangle's side is extended, the extension creates another angle. This angle, called an outside or exterior angle, has a special relationship with the angle adjacent to it. The two angles become supplementary angles, which are angles that have a combined measurement of 180 degrees. You can find the measurements of a triangle's outside angles by working with the measurements of its inside ones.

Protractor

Instructions

- 1
  Select one of the triangle's internal angles.
- 2
  Measure the angle by placing the protractor's center on the angle's vertex, lining up the line labeled "0" with one side of the angle. Record the number along the top that the other leg intersects, using the ascending scale for right opening angles and the descending scale for left opening ones. For example, assume the angle measures 65 degrees.
- 3
  Subtract the angle's measurement from 180 to find the exterior angle. Concluding the example, 180 degrees minus 65 degrees equals 115 degrees.
- 4
  Repeat the steps with the other two angles.

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