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Number theory Difficulty 4.6 AIME Prove it Slovenia

Let xx be a real number such that x+1x+1x + \frac{1}{x} + 1 is a positive integer. Prove that x2+1x2+1x^2 + \frac{1}{x^2} + 1 is a positive integer divisible by x+1x+1x + \frac{1}{x} + 1.

Solution

We have
x2+1x2+1=(x+1x)21=(x+1x+1)(x+1x1). x^2 + \frac{1}{x^2} + 1 = \left(x + \frac{1}{x}\right)^2 - 1 = \left(x + \frac{1}{x} + 1\right)\left(x + \frac{1}{x} - 1\right).
Since x+1x+1x + \frac{1}{x} + 1 is a positive integer, x+1x1x + \frac{1}{x} - 1 is an integer and so is x2+1x2+1x^2 + \frac{1}{x^2} + 1. Since x2+1x2+11x^2 + \frac{1}{x^2} + 1 \ge 1 we conclude that x2+1x2+1x^2 + \frac{1}{x^2} + 1 is a positive integer divisible by x+1x+1x + \frac{1}{x} + 1.

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