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

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