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Romanian Mathematical Olympiad - Final Round

Number theory Difficulty 5.3 AIME, harder Prove it Romania

The positive integer nn is a perfect square. Find the quotient of the division of 20232023 by nn, if the remainder is 223−32⋅n223 - \frac{3}{2} \cdot n.

Solution

Denote by cc the quotient of the division. From the quotient-remainder theorem we obtain 2023=n⋅c+223−32⋅n2023 = n \cdot c + 223 - \frac{3}{2} \cdot n, thus (2c−3)n=3600(2c-3)n = 3600. (1)

The remainder 223−32⋅n223 - \frac{3}{2} \cdot n is a positive integer, therefore nn is even and 0≤223−32⋅n<n0 \le 223 - \frac{3}{2} \cdot n < n, whence we deduce that 90≤n≤14890 \le n \le 148. Because nn is a square, it follows that n=100n = 100 or n=144n = 144. If n=100n = 100, we deduce from (1) that 2c−3=362c - 3 = 36, false, and if n=144n = 144, we obtain the solution c=14c = 14.

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