If then , and you can divide by .
Now subtract the two equations:
Group terms now, so that they cancel,
After canceling, we have:
Now since , divide by on both sides and we are done!
NOTE: This is exactly what clic-clac did, but I have explicitly written down the terms so that you can "see" the solution clearly. Try reading through clic-clac's post, to learn elegant ways of writing proofs involving sums.