I started by saying that gcd and \gcd(a+b,\gcd(a,b)) = d_2
Then I tried to show that \ d_1 \ge d_2, d_1 \le d_2.
I know that \ d_2 | \gcd(a+b, d_1) hence \ d_2 \le d_1 .
How do I prove that \ d_2 \ge d_1 ?
Answer
If \gcd(a,b)=d_1 then a = d_1 x and b= d_1 y, where x,y are integers. Consequently,
\gcd(a+b,\gcd(a,b))=\gcd(d_1(x+y),d_1) = d_1\gcd(x+y,1)=d_1.
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