visit this link and answer the question on screen
imageshack.us/photo/my-images/17/codecogseqn1.gif/
It's trivial:
1/H_{n} - 1/H_{n-1} =
[H_{n-1} - H_{n}]/[H_{n}H_{n-1}] =
-1/[n H_{n}H{n-1}]
imageshack.us/photo/my-images/17/codecogseqn1.gif/
1/H_{n} - 1/H_{n-1} =
[H_{n-1} - H_{n}]/[H_{n}H_{n-1}] =
-1/[n H_{n}H{n-1}]