Let a, b, c, and d be natural numbers. Show that if ab = cd, then a² + b² + c² + d² is not prime. Generalize.
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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: $\displaystyle a^2+b^2+c^2+d^2$ $\displaystyle =a^2+2ab+b^2+c^2-2cd+d^2$ $\displaystyle =(a+b)^2+(c-d)^2$
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