Question

Sin20градусов*(8cos20градусов*cos40 градусов*cos80градусов - 1)

Answer 1

$sin20*(8cos20*cos40*cos80 - 1)=\ 8cos20sin20*cos40*cos80 - sin 20=\ 4*2cos20sin20cos40cos80-sin20=\ 2*2sin40cos40cos80-sin20=\ 2sin80cos80-sin20=\ sin160-sin20=\ sin(180-20)-sin20=\ sin20-sin20=0$