Maths Olympiad Prep

Library / /21 of 120

Number theory Difficulty 4.7 AIME Prove it Saudi Arabia

Let aa and bb be integers such that ab=a2cb2da-b=a^{2} c-b^{2} d for some consecutive integers cc and dd. Prove that ab|a-b| is a perfect square.

Solution

Let d=c+1d = c + 1. The equality ab=a2cb2(c+a)a-b = a^{2} c - b^{2} (c + a) implies
(ab)[c(a+b)1]=b2. (a-b)[c(a+b)-1] = b^{2}.
But c(a+b)1c(a+b)-1 and aba-b are relatively prime. Indeed, if pp is a prime dividing aba-b, then the above equality shows that pp also divides bb, so pp will divide a+b=(ab)+2ba+b = (a-b) + 2b. Hence pp cannot divide c(a+b)1c(a+b)-1. It follows that ab|a-b| is a perfect square as well. A first nontrivial example is 1822=182(3)222(2)18-22 = 18^{2}(-3) - 22^{2}(-2).

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 and solution reproduced as published; topic and difficulty added by this site.