## Geometry: Common Core (15th Edition)

By the definition of congruent, $m\angle ABC=m\angle XYZ$. $m\angle ABC+m\angle XYZ=180\longrightarrow$ is given $m\angle ABC+m\angle ABC=180\longrightarrow$ substitution $2m\angle ABC=180\longrightarrow$ combine like terms $m\angle ABC =90\longrightarrow$ division property of equality $m\angle XYZ=90\longrightarrow$ transitive property of equality