Question

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

Answer 1

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`