## Geometry: Common Core (15th Edition)

The labeled angles form a same-side interior angle pair. The same-side interior angle postulate tells us they are supplementary. $5x+4x=180\longrightarrow$ definition of supplemntary $9x=180\longrightarrow$ combine like terms $x=20\longrightarrow$ division property of equality Substitute to find the angle measures. $5x=5(20)=100$ $4x=4(20)=80$