Maths Olympiad Prep

Library / /46 of 105

Number theory Difficulty 4.9 AIME Prove it United States

Problem:
Determine all integers xx such that the product x(x+1)(x+2)x(x+1)(x+2) is the square of an integer.

Solution

Solution:
The answers are x=0x=0, x=1x=-1, and x=2x=-2, for all of which the product is 0=020=0^{2}.

Assume that there is another solution. We can immediately rule out the case x<2x<-2, as then the product is negative. So xx is a positive integer. Note that the greatest common divisor of xx and x+1x+1 is 11, as two consecutive integers cannot be multiples of the same prime. Likewise, gcd(x+2,x+1)=1\gcd(x+2, x+1)=1 and so gcd(x(x+2),x+1)=1\gcd(x(x+2), x+1)=1. Now x(x+2)x(x+2) and x+1x+1 are relatively prime positive integers whose product is a square, so they must both be squares (any prime dividing either of them must do so to an even power). But x(x+2)=x2+2x=(x+1)21x(x+2)=x^{2}+2x=(x+1)^{2}-1, so now x(x+2)x(x+2) and (x+1)2(x+1)^{2} are two consecutive positive integers which are both squares, which is impossible (the difference between any two consecutive squares a2a^{2} and (a+1)2(a+1)^{2} is 2a+132a+1 \geq 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: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.