cos 10 cos 50 cos 60 cos 70 = root of 3/16
{ all values of cos are in degrees.}
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