(i) By writing $\frac{1}{4 r^{2}-1}$ in partial fractions, find an expression for $\sum_{r=1}^{n} \frac{1}{4 r^{2}-1}$.

[4]

(ii) Hence find the exact value of $\sum_{r=11}^{\infty} \frac{1}{4 r^{2}-1}$.

[2]