Question

cos20cos40cos60cos80=?

Answer 1

Explanation:

Set

cos20cos40cos60cos80=a

multiply both sides by sin20.

sin20cos20cos40cos60cos80=asin20

By double angle formula,

sin20cos20=12sin40

We can continue this pattern.

12sin40cos40cos60cos80=asin20

14sin80cos80cos60=asin20

18sin160cos60=asin20

cos60 is simply 12

So,

116sin160=asin20

However,

sin160=sin20

So, a=116, and we have proven the statement.

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Please mark the answer as brain list answer

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!!!!!