, where0 a, b, c, d are positive integers. Let , , , and perhaps p and q can be positive integers. We will see. Factor the left side: If those expressions in parentheses can be positive integers, they can only be the unequal pair of factors of 9, which are 1 and 9. The p-q² is smaller, so it would be 1 and p+q² is larger so it would be 9: , Add the equations: Subtract the equations: So we now see that p and q can be positive integers. , , , , , , , , , Edwin