Thursday, June 27, 2019

elementary number theory - Prove that gcd(a,b)=gcd(a+b,gcd(a,b))



I started by saying that gcd(a,b)=d1 and gcd(a+b,gcd(a,b))=d2



Then I tried to show that  d1d2,d1d2.



I know that  d2|gcd(a+b,d1) hence  d2d1.



How do I prove that  d2d1 ?


Answer




If gcd(a,b)=d1 then a=d1x and b=d1y, where x,y are integers. Consequently,
gcd(a+b,gcd(a,b))=gcd(d1(x+y),d1)=d1gcd(x+y,1)=d1.


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