how to prove: every two numbers gcd can be divided by this two numbers any common divisor for example, I have two numbers: 36 and 48. their gcd is 12. 12 can be divided by any common divisors of these two numbers: 1, 2, 3, 4, 6, 12. Is there a general proof for this? Thanks!
 A: Let $\gcd(m,n) = d$.  Then $m= d*m'$ and $n=d*n'$ for some integers $m',n'$.
Now if $m', n'$ had any factor  in common (other than $1$), and we called that factor $f$, then $f*d$ would be a common divisor that is larger then $d=\gcd(m,n)$.  So we can safely conclude $m',n'$ (the remains as it were) have no factors in common[$*$].
If $a$ is a common factor of $m$ and $n$ it can not share any factors (other than $1$) with either $m'$ nor $n'$ as $m',n'$ have no factors (other than $1$) in common.  
Now $a|m$ so $\frac ma = \frac {dm'}a = k$ is an integer. $\frac na = \frac {dn'}a = j$ is an integer.  If $a\not \mid d$ then $a $ will have factors that do not divide $d$ and must therefore divide $m'$ and $n'$.  But $m'$ and $n'$ have no factors in common.
That's it.
[$*$] Note, it would be a mistake to conclude that $m'$ and/or $n'$ will have no factors in common with $d = \gcd(m,n)$ or with $a$.  It is possible that one of $m'$ or $n'$ will have some factors in common with $d$ or with $a$. but it is impossible that both $m'$ and $n'$ will have factors in common.
A: Claim: If $a\vert b$ and $b\vert c$, then $a\vert c$.
Proof: $c=kb=k(ma)=(km)a$.
A: The definition of $d=\gcd(a,b)$ is that it is the positive generator of the ideal $(a,b)\subset\mathbf Z$.
So there exist integers $u,v$ such that $ua+vb=d$. Now if $c$ divides both $a$ and $b$, we can write $a=ca'$, $b=cb'$ -$a',b'\in\mathbf Z$. We deduce at once
$$d=uca'+vcb'=c(ua'+vb').$$
