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I'm a physicist taking elementary number theory so while I've had a class discussing proofs in the past, my experience with rigorous mathematical proofs is very limited. I've got a problem and I'm not sure how many things I need to show to adequately complete it. The question is:

Prove that if $d$ is a common divisor of $a$ and $b$, then $d = gcd(a,b)$ if and only if $gcd(a/d,b/d) = 1$.

The math itself is very simple. But what I actually need to show is less clear to me. I know iff statements need to be shown both ways ($A\Rightarrow B$ and $B\Rightarrow A$ since it's essentially an equality statement), so would I need to show both directions for that and THEN assume 'if $d$ is a common divisor, then...'? Or should I start with that assumption and do something extra later on?

Much appreciation for the help.

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    In this case, every situation we are considering has $d$ being a common divisor of $a$ and $b$. With that knowledge, we wish to show that $d=gcd(a,b)$ implies $gcd(a/d,b/d)=1$ as well as showing that $gcd(a/d,b/d)=1$ implies $d=gcd(a,b)$. Note that these statements need not necessarily imply one another in the case that $d$ is not a common divisor of each. – 2017-02-04
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    Very well explained. Thanks for your quick response. – 2017-02-04

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You can prove a proposition of the form 'If A then (B if and only if C)' by doing these two things: First assume A and B and prove C. Then assume A and C and prove B.

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    That makes sense. Thank you very much. – 2017-02-04
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Show that $\gcd(a, b)$ $=$ $d$ if and only if $\gcd(a/d, b/d)$ $=$ $1$

What is the gcd? The gcd stands for greatest common divisor. Notice the word in bold. So the gcd of $a$ and $b$ is $d$ means there is no other divisor $x$ in common with $a$ and $b$, that is $x>d$. Therefore since $d$ is the gcd of $a$ and $b$, $a/d$ and $b/d$ must be relatively prime (their gcd is $1$). Otherwise, the gcd would be $hd$ with $h$ $>$ $1$. Hence $hd$ $>$ $d$.