cos 10 cos 50 cos 60 cos 70 = root of 3/16

{ all values of cos are in degrees.}

Ayush Kumar Jha , Meritnation Expert added an answer, on 6/2/13

We have to evaluate-

=cos10 cos50 cos60 cos70

= cos50 cos10 cos70

The above expression can be manipulated as-

= cos(60-Q) cosQ cos(60+Q)

where, Q=10 such that

cos(60-Q) cosQ cos(60+Q)=

So,

= cos30

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