Question

sin 20 (8 cos 20 cos 40 cos 80 - 1)= ?

Answer 1

sin 20 (8 cos 20 cos 40 cos 80 - 1)= (2sin 20 cos 20) * 4 cos 40  cos 80 - sin 20 =

 = sin 40 * 4 cos 40 cos 80 - sin 20 = (2 sin 40 cos 40) * 2cos 80 - sin 20 =

= 2sin 80 cos 80 - sin 20 = sin 160 - sin 20 = 2sin 70 cos 90 = 0