ISI2018-PCB-A3

Question

Let $n,r\ $and$\ s$ be positive integers, each greater than $2$.Prove that $n^r-1$ divides $n^s-1$ if and only if $r$ divides $s$.

Comments

This might help: https://math.stackexchange.com/questions/128007/xa-1-divides-xb-1-if-and-only-if-a-divides-b