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Confusing math question

Discussion in 'DAT Discussions' started by wired202808, Jul 24, 2011.

  1. wired202808

    wired202808 Membership Revoked
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    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?
     
  2. buckey

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    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.
     
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  3. Hope30

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    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.
     

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