Let x be a real number such that x+x1+1 is a positive integer. Prove that x2+x21+1 is a positive integer divisible by x+x1+1.
Solution
We have x2+x21+1=(x+x1)2−1=(x+x1+1)(x+x1−1). Since x+x1+1 is a positive integer, x+x1−1 is an integer and so is x2+x21+1. Since x2+x21+1≥1 we conclude that x2+x21+1 is a positive integer divisible by x+x1+1.
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