What is a Supplementary Angle?

A pair of supplementary angles is two angles whose measures sum to 180º. An example of a pair of supplementary angles is a 110º angle and a 70º angle.

Let’s take a look at an example:

Here is a straight line, let’s call it Z, it is 180º. Another line, let’s call it Y, intersects Z and creates two angles. The one (b) is obviously bigger than 90º and is therefore obtuse. The other (a) is smaller than 90º and is therefore acute. These two angles are complementary, because they will always add up to 180º.

Let’s try our hand at a few practice problems. Here are two angles. One is 34 degrees, and we can see they are supplementary since they are on a straight line. So, if we know that supplementary angles add up to 180 degrees, we can simply subtract 34 from 180 to find the angle x. In this case, x = 146 degrees

One last one. Can you figure this one out by yourself? I’ll give you a few seconds. The answer is y = 12 degrees. Because 180 – 168 = 12. Make sense?

I hope that helps. Thanks for watching this video lesson. Until next time, happy studying.

Supplementary Angle Practice Questions