1) Chứng minh:a. $sin\frac{2\pi }{7}$ + $sin\frac{4\pi }{7}$ +$sin\frac{6\pi }{7}$ = $\frac{1}{2}$$cot\frac{\pi }{14}$.
b. $cos\frac{\pi }{11}$ + $cos\frac{3\pi }{11}$ +$cos\frac{5\pi }{11}$ + $cos\frac{7\pi }{11}$+ $cos\frac{9\pi }{11}$ =$\frac{1}{2}$.