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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.2012-03-05

1 Answers 1

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Assume not.

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...

Does that help?

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    Huzzah! Thank you fo$r$ the help!!2012-03-05