Question

How do you find Sin x/2 if cos x=7/9 using the half-angle identity?

Answer 1

`+- 1/3`

Explanation:

Apply the trig identity:
`cos 2a = 1 - 2sin^2 a`. We get:
`cos x = 7/9 = 1 - 2sin^2 (x/2)`
`2sin^2 (x/2) = 1 - 7/9 = 2/9`
`sin^2 (x/2) = 1/9`
`sin (x/2) = +- 1/3`