## Geometry: Common Core (15th Edition)

$m\angle1=100$ $m\angle2=70$
$\angle 1$ forms a same-side interior angle pair with the angle marked $80^o$. The same-side interior angle postulate tells us these two angles are supplementary. $m\angle1+80=180$ $m\angle1=100$ $\angle 2$ forms an alternate interior angle pair with the angle marked $70^o$. The alternate interior angle theorem tells us these two angles are congruent, so have the same measure.