# Prove: If $g\circ f$ is $1-1$ and $f$ is onto, then $g$ is $1-1.$

Let $f:A \to B, g:B \to C$. I don't really know how to prove this but I understand what it means.

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As a general hint with these types of problems see what happens if you assume the conclusion is false. –  Chris Janjigian Mar 5 '12 at 4:54

Then there exist $a,b$ with $a \neq b$ s.t. $g(a) = g(b)$. $f$ onto means there exist $c,d$ s.t. $f(c) = a, f(d) = b$. But then...
$a \neq b$, right? –  The Chaz 2.0 Mar 5 '12 at 5:26