## Geometry: Common Core (15th Edition)

$m\angle ACD=90$
Use the definition of a linear pair, given that $\angle FCA$ is a right angle: $m\angle FCA+m\angle ACD=180$ $90+m\angle ACD=180$ Subtract 90 from each side. $90+m\angle ACD-90=180-90$ $m\angle ACD=90$