Question

What is `cos(theta/2)` in terms of trigonometric functions of a unit `theta`?

Answer 1

`+- sqrt((1 + cos t)/2)`

Explanation:

Apply the trig identity: `cos 2a = 2cos^2 a - 1`.
`2cos^2 (t/2) = 1 + cos t`
`cos^2 (t/2) = (1 + cos t)/2`
`cos (t/2) = +- sqrt((1 + cos t)/2)`