Saturday, 24 August 2019

elementary number theory - If gcd(ab,c)=d and c|ab then c=d



For all positive integers a, b, c and d,

if gcd and c | ab, then c = d.



Need help proving this question, I know that abx + cy = d for integers x,y
and that c|ab can be c=q\cdot ab but I'm not sure how to apply these facts or if they're even useful in this proof.



Any help to get me started would be great.


Answer



c|ab means that ab=q\cdot c, not the other way around!



Therefore, you have ab=q\cdot c and ab x + c y = d which you can rewrite into qcx + cy = d\\ c(qx+y) = d



Can you continue from here?


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