## Geometry: Common Core (15th Edition)

$\angle ADB$
From the Angle Addition Postulate, the angle formed by the noncommon sides of adjacent angles has the same measure as the sum of the adjacent angles. In the given example, $\overline{DC}$ is the common side, so $\overline{DA}$ and $\overline{DB}$ are the sides of the straight angle $\angle ADB$.