Tinh$a) S=2^{4}.sin\frac{\pi}{24}.sin\frac{5\pi}{24}.sin\frac{7\pi}{24}.sin\frac{11\pi}{24}$$b) S= sin\frac{\pi}{30}.sin\frac{7\pi}{30}.sin\frac{13\pi}{30}.sin\frac{19\pi}{30}.sin\frac{25\pi}{30}$$c) S = tan a + \frac{1}{2}tan \frac{a}{2} + ... + \frac{1}{2^n}tan \frac{a}{2^{n}}$$d) S = (2.cos a -1)(2.cos 2a -1)...(2.cos(2^{n-1}a)-1)$