## Basic College Mathematics (10th Edition)

To be complementary the sum of the two angles must be equal to 90$^{\circ}$. Since acute angles are less than 90$^{\circ}$ it is possible for them to be complementary. For example 20$^{\circ}$ and 70$^{\circ}$ are both acute and complements of each other since their sum is 90$^{\circ}$.