Maths Olympiad Prep

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Number theory Difficulty 4.8 AIME Find the answer

Find all positive integer solutions (m,n)(m, n) to the following equation: m2=1!+2!++n! m^{2}=1!+2!+\cdots+n!

A number or a short expression. Spacing and $ signs are ignored.

Solution

A square must end in the digit 0,1,4,5,60,1,4,5,6, or 9 . If n4n \geq 4, then 1!+2!++n1!+2!+\cdots+n ! ends in the digit 3 , so cannot be a square. A simple check for the remaining cases reveals that the only solutions are (1,1)(1,1) and (3,3)(3,3).

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