Ta CM đc $S=sin\frac{\pi }{14}+cos\frac{\pi }{7}-cos\frac{2\pi }{7}$
Nhân cả 2 vế cho $cos\frac{\pi }{14}$
$\Leftrightarrow S.cos\frac{\pi }{14}=\frac{1}{2}sin\frac{\pi }{7}+\frac{1}{2}(cos\frac{3\pi }{14}+cos\frac{\pi }{14})-\frac{1}{2}(cos\frac{3\pi }{14}+cos\frac{5\pi }{14})$
$\Leftrightarrow S.cos\frac{\pi }{14}=\frac{1}{2}(sin\frac{\pi }{7}+cos\frac{\pi }{14}-cos\frac{5\pi }{14})$
$\Leftrightarrow S.cos\frac{\pi }{14}=\frac{1}{2}(sin\frac{\pi }{7}+cos\frac{\pi }{14}-sin\frac{\pi }{7})$
$\Leftrightarrow S=\frac{1}{2}$