## Geometry: Common Core (15th Edition)

Let a be the measure of the angle. By the definition of supplement, the measure of the supplement is $180-a$. Convert the sentence into an expression. $a=\frac{1}{3}(180-a)$ multiply each side by 3 $a\times3=\frac{1}{3}(180-a)\times3$ $3a=180-a$ add a to each side $3a+a=180-a+a$ $4a=180$ divide each side by 4 $4a\div4=180\div4$ $a=45$