Cos70+8cos20cos40cos80=2cos^2 35

Question

Cos70+8cos20cos40cos80=2cos^2 35

Answer 1

Сos70+8sin20cos20cos40cos80/sin20=
=cos70+4sin40cos40cos80/sin20=
=cos70+2sin80cos80/sin20=
=cos70+sin160/sin20=
=cos70+sin(180-20)/sin20=
=cos70+sin20/sin20=
=cos70+1=
=2cos²35