Confusing math question

[wired202808]

Find the value of Cos -120:

The answer is -cos 60.

Here is where im confused: doesnt this formula cos -A = Cos A; ensure that the answer has to be positive?

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[buckey]

cosine of -120 is in the third quadrant, at 240 degrees ccw from 0 or 120 cw from 0. at this point it is 60 degrees from the third quadrant reference angle of 180. cosine is negative in the third quadrant so the answer is -cos 60.

 

[Hope30]

Just remember "All Students Take Calculus." First quadrant- All are (+). Second quadrant- sine is (+). Third quadrant- tangent is (+). Fourth quadrant- cosine is (+). Using this, you can answer basically any questions related to trig.