Maths Olympiad Prep

Library / /81 of 841

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. Fractions can be typed as 3/2, and spacing doesn't matter.

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).

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.