Find the measure of an angle, which is equal to its supplement.

Finding the value of x:

1. Two angles are said to be supplementary if the sum of their measures is equal to 180^o.
2. The supplement of an angle can be found by subtracting that angle from 180^o.

Let x^o be a supplement of an angle,

Now in the question is it saying that the supplement of x is equal to x.

$$\begin{gathered} x^o={(180-x)}^o \\ x+x=180 \\ 2x=180 \\ x={90}^o \end{gathered}$$.

Hence, the angle which is equal to its supplement is 90°.