Show cos 20 * cos 40 * cos 80 = 1/8

cos 20 cos 40 cos 80 =

= (2 sin 20 cos 20 cos 40 cos 80) / 2 sin 20 =

(multiple and divide by 2 sin 20)

= (sin 40 cos 40 cos 80) / 2 sin 20 =

= (1/2 sin 80 cos 80) / 2 sin 20 =

= (sin 80 cos 80) / 4 sin 20 =

= 1/2 sin 160 / 4 sin 20 =

= sin 160 / 8 sin 20 = (and since: sin 160 = sin 20)

= 1 / 8