Question

Cos 20 cos 40 cos 60 cos 80 = 1/2

Answer 1

given,
cos 20 cos 40 cos 60 cos 80
= cos 20 cos 40 cos 60 × sin 10
= cos 20 cos 40 cos 60 × $\frac{2sin 10 cos 10}{2cos 10}$
= cos 40 cos 60 × $\frac{sin 20 cos 20}{2cos 10}$
= cos 60 × $\frac{sin 40 cos 40}{4cos 10}$
= cos 60 × $\frac{sin 80}{8cos 10}$
= $\frac{cos 10}{16cos 10}$
= $\frac{1}{16}$

Answer 2

DEAR STUDENT,

Kindly check the attached papers for a detailed solution along with step by step elaboration to reach our final solution that is our final desired or required answer to this query that is completely proved.

$\boxed{\bf{\underline{L.H.S. = R.H.S.}}}$

Which is the required proof or solution process for this type of query.

Hope this helps you and solves your doubts for applying trigonometric identifies to reach a proper solution or a proof!!!!!