## Precalculus (6th Edition)

----------- $y=f(x)=4x+20$ Assume that $f(a)=f(b)$ $\ \ \Rightarrow \ \ 4a+20=4b+20\qquad /-20$ $\ \ \Rightarrow \ \ 4a=4b\qquad /\div 4$ $\ \ \Rightarrow \ \ a=b$ $f$ is one-to-one