1. ## question and trial

$
\begin{array}{l}
prove{\rm that} \\
{\rm gcd(a,b)} \le {\rm a - b , a} \succ {\rm b} \\
{\rm solution:} \\
{\rm note}\;{\rm that}\;{\rm gcd(a,b) = gcd(a, - b)} \\
\Rightarrow {\rm d\backslash a , d\backslash - b} \Rightarrow {\rm d\backslash a + - b} \\
{\rm this}\,\;{\rm means}\;{\rm that}\;{\rm d} \le {\rm a - b } \Rightarrow {\rm gcd(a,b)} \le {\rm a - b }{\rm .} \\
\\
\end{array}$

2. Is there a question here, or are you just wanting someone to look over your answer? It looks good to me...