Решите пожалуйста  cos20cos40cos80=

[latex]cos20cos40cos80=\frac{1}{2sin20}\cdot (2sin20cos20)cos40cos80=\\\\=\frac{1}{2sin20}\cdot sin40cos40cos80=\frac{1}{4sin20}\cdot (2sin40cos40)cos80=\\\\=\frac{1}{4sin20}\cdot sin80cos80=\frac{1}{8sin20}\cdot 2sin80cos80=\frac{sin160}{8sin20}=\\\\=\frac{sin(180-20)}{8sin20}=\frac{sin20}{8sin20}=\frac{1}{8}[/latex]