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Ukrainian National Mathematical Olympiad

Algebra Difficulty 4.5 AIME Prove it Ukraine

Find all natural numbers nn that satisfy the inequalities:
−46≤202346−n≤46−n. -46 \le \frac{2023}{46-n} \le 46-n.

Solution

Let x=46−nx = 46 - n and find the corresponding integer values of xx that satisfy the condition −46≤2023x≤x-46 \le \frac{2023}{x} \le x. We consider two cases.

Case 1. x>0x > 0. The left inequality holds for all the mentioned values of xx, and the right one can be rewritten as:
2023≤x2⇒x≥45⇒46−n≥45⇒n≤1⇒n=1. 2023 \le x^2 \Rightarrow x \ge 45 \Rightarrow 46 - n \ge 45 \Rightarrow n \le 1 \Rightarrow n = 1.

Case 2. x<0x < 0. The inequality can be rewritten as:
x2≤2023≤−46x⇒−44≤x≤44 and x≤−44⇒x=−44⇒46−n=−44⇒n=90. x^2 \le 2023 \le -46x \Rightarrow -44 \le x \le 44 \text{ and } x \le -44 \Rightarrow x = -44 \Rightarrow 46 - n = -44 \Rightarrow n = 90.

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