I tried this last night and i'm looking at it this mornng. I'm still stuck on a bit of it:
My method is like this:Observe that:
Guess the general law and prove it by induction.
Hence true for n=1.
Assume true for n=k:
At this point it's clear that the LHS does not equal the right RHS. This implies that my initial expression for the sum is incorrect. However, it works for n=1 and n=2 so I think it's right.
Can anyone see what i'm doing wrong?