pls solve this problem for me as m not also to solve it...

question: suppose a and b are relatively prime. prove that ab and a+b are relatively prime...

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- Aug 2nd 2012, 07:56 AMlokeshkumarguptacoprime problem
pls solve this problem for me as m not also to solve it...

question: suppose a and b are relatively prime. prove that ab and a+b are relatively prime... - Aug 3rd 2012, 01:39 AMkraj8995Re: coprime problem
suppose a=3

b=5

ab=5*3=15

a+b=8

so a+b is not a relatively prime. - Aug 3rd 2012, 01:44 AMemakarovRe: coprime problem
- Aug 3rd 2012, 03:55 AMlokeshkumarguptaRe: coprime problem
two numbers a,b are said to be coprime if gcd(a,b)=1

- Aug 3rd 2012, 03:57 AMDevenoRe: coprime problem
one could suppose that a counter-example existed. if so then since d > 1 divides ab and a+b, there is some prime p that divides d. but p, being prime, must divide a or b.

suppose it were a, that is a = kp. since p also divides a+b (since d does), a+b = rp. hence b = a+b - a = rp - kp = (r-k)p, so p divides b.

or, suppose it were b, so that b = mp. then a = a+b - b = rp - mp = (r-m)p, and so a is likewise divisible by p.

either way, assuming that gcd(ab,a+b) > 1 leads to a prime p dividing a and b. but gcd(a,b) = 1, so how can this be possible?