Q. If $S _{\infty}=\frac{1}{1 \cdot 3 \cdot 5 \cdot 7 \cdot 9}+\frac{1}{3 \cdot 5 \cdot 7 \cdot 9.11}+\frac{1}{5 \cdot 7 \cdot 9 \cdot 11 \cdot 13}$ $+\ldots$, then the value of $\frac{1}{120 S _{\infty}}$ is
Sequences and Series
Solution: