Answers

2016-03-02T13:37:37+05:30
We have a formula that cosx×cos(60-x)×cos(60+x)
=1\4cos3x
cos20×cos40×cos80=
cos20×cos(60-20)×cos(60+20)=
[by using the before formula]
=1\4cos3(20)[cos60=1\2]
=1\4cos60=1\4×1\2=1\8
hence proved
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