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

1 Answer
Trevor Ryan.
Oct 30, 2015

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.

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