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Algebra Difficulty 4.6 AIME Prove it Slovenia

Find all integers nn, such that the equation x2+nx+n+5=0x^2 + nx + n + 5 = 0 has only integer solutions.

Solution

Let x1x_1 and x2x_2 be the integer solutions of this quadratic equation. We may assume that x1x2x_1 \le x_2. By Viete's formulas we have x1+x2=nx_1 + x_2 = -n and x1x2=n+5x_1x_2 = n + 5. Adding both identities together we get x1x2+x1+x2=5x_1x_2 + x_1 + x_2 = 5, so

(x1+1)(x2+1)=6.(x_1 + 1)(x_2 + 1) = 6.

Since 6=16=23=(3)(2)=(6)(1)6 = 1 \cdot 6 = 2 \cdot 3 = (-3) \cdot (-2) = (-6) \cdot (-1), we have four possible cases to consider. In the first case we have x1=0x_1 = 0, x2=5x_2 = 5, in the second case x1=1x_1 = 1, x2=2x_2 = 2, in the third case x1=4x_1 = -4, x2=3x_2 = -3 and in the final case x1=7x_1 = -7, x2=2x_2 = -2. Using the identity x1+x2=nx_1 + x_2 = -n we see that nn is equal to 5,3,7-5, -3, 7 or 99.

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