VII OBM

$a$, $b$, $c$, $d$ are integers with $ad\ne bc$. Show that $\frac{1}{(ax+b)(cx+d)}$ can be written in the form $\frac{r}{ax+b}+\frac{s}{cx+d}$. Find the sum $$\frac{1}{1\cdot 4}+\frac{1}{4\cdot 7}+\frac{1}{7\cdot 10}+\cdots +\frac{1}{2998\cdot 3001}$$