Question

How do you find the exact value of `cos -120`?

Answer 1

`cos-120^@=-1/2=-0,5`

Explanation:

`cos(-120^@)=cos120^@`

`=cos(180^@-60^@)`

`=-cos60^@`

`=-1/2`.

Note: Above I have used the `180^@` rule.
Alternatively I could also have used the `90^@` rule, or the compound angle formulae for either sin or cos, or a number of other possible methods as well, and each would result in exactly the same final answer.
Since the question did not explicitly specify which method to use, or how to express the final answer, any valid method will suffice.