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

Dec 24, 2014

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