This is probably simple, but as I'm studying for a test I thought I'd better post anyway:
Prove by Mathematical Induction that 5^n + 2 x 11^n is a multiple of 3.
i. Prove true for n = 1
so 5^1 + 2 x 11^n = 77.
77/3 doesn't give a whole number however, so do you continue with the proof or is...