# Math Help - Prime # Proof

1. ## Prime # Proof

Let a, b, c, and d be natural numbers. Show that if ab = cd, then a² + b² + c² + d² is not prime. Generalize.

2. Originally Posted by MATNTRNG
Let a, b, c, and d be natural numbers. Show that if ab = cd, then a² + b² + c² + d² is not prime. Generalize.
I think the start goes like this:

$a^2+b^2+c^2+d^2$

$=a^2+2ab+b^2+c^2-2cd+d^2$

$=(a+b)^2+(c-d)^2$