Let \(x\) be two equal angles of its supplement.
Therefore, \(x + x = 180\degree\) [Supplementary angles]
\(\Rightarrow 2x = 180\)
\(\Rightarrow x = \frac{180\degree}{2}= 90\degree\)
Thus, \(90\degree\) is equal to its supplement.
Read more: Supplementary and Complementary Angles