I think it is by contradiction (i suppose you could show the gcd of [itex](a^n - b^n , a^n + b^n )[/itex] is 1 or 2 ) viz,
let d be the gcd clearly then , d must divide the sum (and the difference) of the two , [tex]d | a^n + b^n + a^n - b^n[/tex]
[tex]d | 2a^n[/tex]
which implies, [itex]d|2,[/itex] [itex]d|a^n[/itex] this last result shows d is either 1 or 2 , thus if the gcd of the two is 2 or 1, no integers a,b, c >1 can exist to satisfy the requirement (there are no numbers a,b, c> 1 that can divide in that manner) though I am only into number theory as a hobby, i might not be quite on the mark, ;-) ,
good luck