Cos 2п/7 умножить на cos 3п/7 умножить на cos 6п/7

[latex]cos\frac{2\pi }{7}cos\frac{3\pi }{7}cos\frac{6\pi}{7}=cos\frac{2\pi}{7}\cdot \frac{sin\frac{3\pi}{7}\cdot cos\frac{3\pi}{7}}{sin\frac{3\pi}{7}}\cdot cos\frac{6\pi}{7}=\\\\=cos\frac{2\pi}{7}\cdot \frac{sin\frac{6\pi}{7}}{2sin\frac{3\pi}{7}}\cdot cos\frac{6\pi}{7}=cos\frac{2\pi}{7}\cdot \frac{1}{2sin\frac{3\pi}{7}}\cdot \frac{1}{2}sin\frac{12\pi }{7}= [/latex] [latex]=\frac{cos\frac{2\pi}{7}\cdot sin\frac{12\pi}{7}}{4sin\frac{3\pi}{7}}= \frac{2cos\frac{10\pi}{7}\cdot sin\pi }{4sin\frac{3\pi}{7}} =0\; ,\; tak\; kak\; \; sin\pi =0.[/latex]