## Geometry: Common Core (15th Edition)

$\angle ADB\text{ and }\angle BDC\text{ are complementary.}$
$\text{Complementary}$ angles are two angles whose sum is $90^{\circ}$. Since $\angle ADB\text{ and }\angle BDC$ are adjacent and form a right angle, they fit this definition, and we can consider them to be $\text{complementary}$.