Each of x and y is a positive integer such that: x² – 5y² = 1.

Prove that x*y is always divisible by 12.

(In reply to re: Possible solution by Harry)

Thanks!

I wasn't sure if the extra step was needed. Indeed the stronger solve integer 4n-5y^2=1 also produces no integer solutions; I'll add something to my earlier post to round off the proof.

Edited on December 4, 2010, 12:31 am

Posted by broll on 2010-12-04 00:15:06