Prove that (a , b) = 1 if and only if (a + b , ab) = 1.
Any help would be great!
Thanks
I start you out. Say there is a prime $\displaystyle p$ dividing both $\displaystyle a+b$ and $\displaystyle ab$. Now $\displaystyle p|ab$ means $\displaystyle p|a$ or $\displaystyle p|b$. WLOG say $\displaystyle p|a$. However, $\displaystyle p|(a+b) \implies p|b$. And so there is a prime dividing $\displaystyle a\text{ and }b$ which contradicts that $\displaystyle (a,b)=1$.
Or if you don't want to use primes and the FTA at all...
Suppose gcd(a,b)=1, let d=gcd(a+b,ab). Since d|ab and gcd(a,b)=1, then d|a or d|b (but not both, unless d=1). Assume (WOLG) d|a. Now since d|a and d|a+b then d|b. But gcd(a,b)=1, so d=1.
Conversely, suppose gcd(a+b,ab)=1. Then there exists x,y such that (a+b)x+aby=1=ax+bx+aby=ax+b(x+ay)=ax+by'. Since there exists x,y' such that ax+by'=1, then gcd(a,b)|1. But this means gcd(a,b)=1.
Your right I did it in the wrong order. My bad. Here's the correct version of that part.
Suppose $\displaystyle gcd(a,b)=1$, let $\displaystyle d=gcd(a+b,ab).$
$\displaystyle d|a+b, gcd(a,b)=1 \implies d\not{|} a \ \ AND\ \ d \not{|}b\ \ OR \ \ d=1.$ Suppose $\displaystyle d\not{|} a \ \ AND \ \ d \not{|}b$. Then $\displaystyle d\not{|}ab$. But that's impossible, since $\displaystyle gcd(a+b,ab)=d.$ Thus $\displaystyle d=1.$