Maths Olympiad Prep

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Algebra Difficulty 4.5 AIME Prove it United States

Problem:

Do there exist four consecutive positive integers whose product is a perfect square?

Solution

Solution:

The answer is no. If x1x \geq 1 is an integer,
x(x+1)(x+2)(x+3)=[x(x+3)][(x+1)(x+2)]=[x2+3x][x2+3x+2]=[(x2+3x+1)1][(x2+3x+1)+1]=(x2+3x+1)21. \begin{aligned} & x(x+1)(x+2)(x+3) \\ & = [x(x+3)] \cdot [(x+1)(x+2)] \\ & = \left[x^2 + 3x\right] \cdot \left[x^2 + 3x + 2\right] \\ & = \left[\left(x^2 + 3x + 1\right) - 1\right] \cdot \left[\left(x^2 + 3x + 1\right) + 1\right] \\ & = \left(x^2 + 3x + 1\right)^2 - 1. \end{aligned}
Therefore the product of four consecutive positive integers is always one less than a square, and therefore cannot be a square since two positive squares cannot differ by 11.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.