Prove by induction that is a multiple of for all values of .
All help much appreciated.
Assume that holds true for n = k.
is divisible by 13.
Prove that it holds true for n = k + 1:
In this final sum, the first addend is obviously divisible by 13, and since what's inside the parentheses is divisible by 13, so is the second addend. Thus the final sum is divisible by 13.