IMO 2001 Q6 #
Let $a$, $b$, $c$, $d$ be integers with $a > b > c > d > 0$. Suppose that
$$ ac + bd = (a + b - c + d) * (-a + b + c + d). $$
Prove that $ab + cd$ is not prime.
Archive.Imo.Imo2001Q6
Let $a$, $b$, $c$, $d$ be integers with $a > b > c > d > 0$. Suppose that
$$ ac + bd = (a + b - c + d) * (-a + b + c + d). $$
Prove that $ab + cd$ is not prime.