Question

How do you find the value of `cos(4pi/3)` using the double angle identity?

Answer 1

`-1/2`

Explanation:

Use the trig identity:
`cos 2a = 2cos^2 a - 1`
`cos ((4pi)/3) = 2cos^2 ((2pi)/3) - 1`
Trig table gives: `cos ((2pi)/3) = -1/2`. There for:
`cos ((4pi)/3) = 2(1/4) - 1 = 1/2 - 1 = -1/2`