How do you use the half angle identity to find cos 15?

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Answer 1 · 2014-12-24T13:23:23

I guess you mean #15°#. If it is so:

Consider:
#cos(15°)=cos((30°)/2)#

The half angle identity can be written as:

#cos(theta/2)=+-sqrt((1+cos(theta))/2#

giving:

#cos(15°)=cos((30°)/2)=+-sqrt((1+cos(30°))/2)=+-0.966#