# Integrated Summation

**Calculus**Level 5

Consider the following sequence of variables defined as follows:

\[\displaystyle a_n = \int\limits_0^\frac{\pi}{2} \frac{1- \cos 2nx }{1- \cos 2x } \text{d}x \]

If

\[\displaystyle S = \sum_{n=1}^\infty \frac{1}{\sum_{r=1}^n a_r a_{r+1} a_{r+2} a_{r+3}} \]

Evaluate \(\displaystyle 24\pi ^4 \cdot S\).