# Math Help - Series

1. ## Series

$\sum_{r=1}^{20} \frac{1}{(r+3)(r+6)}$

I'll never understand Series.

2. $\frac{1}{(k+3)(k+6)} = \frac{1}{3}\left( \frac{ 1 }{k+3} - \frac{1}{k+6}\right)$.

3. Originally Posted by Silver
$\sum_{r=1}^{20} \frac{1}{(r+3)(r+6)}$

I'll never understand Series.
If you're still stuck after reading TPH's reply, read Telescoping series - Wikipedia, the free encyclopedia