Question

cos20°×cos40°×cos60°×cos80°=1/16

Answer 1

costhita.cos(60+thita )cos(60-thita)= 1/4cos 3thita (it is the formula)
(thita = 20)
=1/4 cos 3×20
=1/4cos 60
=1/4×1/2. (cos 60= 1/2)
=1/8×cos60
=1/8×1/2
=1/16