Question

How do you evaluate `cos ((5pi)/3)`?

Answer 1

`1/2`

Explanation:

Apply the trig identity: cos 2a = 2cos^2 a - 1
`cos ((5pi)/3) = 2cos^2 ((5pi)/6) - 1.`
Trig table gives --> `cos ((5pi)/6) = -sqrt3/2`
Therefor,
`cos ((5pi)/3) = 2(3/4) - 1 = 3/2 - 1 = 1/2`