K $>$ L $>$ M $>$ N are positive integers such that$,$ KM $+$ LN $=$ (K$+$L$-$M$+$N)($-$K$+$L$+$M$+$N)$.$ Prove that KL $+$ MN is not prime.
I'm stumped :/
K $>$ L $>$ M $>$ N are positive integers such that$,$ KM $+$ LN $=$ (K$+$L$-$M$+$N)($-$K$+$L$+$M$+$N)$.$ Prove that KL $+$ MN is not prime.
I'm stumped :/
This is problem 6 from the 2001 Intnernational Mathematics Olympiad. That should be enough for you to google the answer (or perhaps shame you into not doing so).
$L-M$ is positive so $K+L-M+N $ is positive, and hence the second factor $-K + L + M +N $ is also positive. Now assume $ KL + MN $ is prime. Then one of the factors is $1$ and the other is $KL + MN.$ Since $ K+L - M + N > K+N > 1 $ we have $ K + L - M + N = KL + MN.$
This rearranges to $ L-M = K(L-1) + (M-1)N$ which is absurd since the left side is less than $L$ while the right hand side is at the very least $K > L.$ Thus the assumption leads to a contradiction and the number is prime.
EDIT: Now that I realize this is a Problem 6 from an IMO competition, I suspect I've made a grave error in my answer above, but I can not see it. I would be grateful if someone could point it out.