so I'm struggling with this one question or better any question of this type: 

You are given that $$\frac{3}{(5+3x)(2+3x)}\equiv \frac{1}{2+3x}-\frac{1}{5+3x}$$ use this result to find $$\sum\_{r=1}^n \frac{1}{(5+3x)(2+3x)}$$ where n=100, giving your answer as an exact fraction.

I would know exactly what to do if they asked for the $$\sum\_{r=1}^n \frac{3}{(5+3x)(2+3x)}$$ because then I would simply use the method of differences with $$\sum\_{r=1}^n (\frac{1}{2+3x}-\frac{1}{5+3x})$$ but here I have no clue. I looked through all my textbooks and revisionguides etc. but the examples in there are only of the kind I know how to do. Please help I have only two weeks until my exam and I'm kinda freaking out...