Figure 1
Exterior angles are angles between an extended side of a triangle and its adjacent side. They are not actually inside the triangle, but can still be very useful in finding the angles inside the triangle.
Figure 2

Two angles on a straight line are called supplementery angles. The sum of these angles is always 180 degrees, as shown in figure 2. Exterior angles are always on straight lines, with its supplementery angles inside the triangle. This information can help us calculate the angle inside the triangle.

You can also use exterior angles to find the size of the angles in the triangle not adjacent to it. As shown in figure 1, the sum two angles of a triangle that are not adjacent to the exterior angle will always be equivalent to the exterior angle. Now that you know this, test your knowledge with these questions.

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