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Math Help - easy trigo question

  1. #1
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    easy trigo question

    Id like to.confirm, cos(180-a) is -cosa right? I was reviewing my.past.notes and realised one question where I had stated that cos(180-a)=cosa. It goes.like this " given that tana=3/4 and sinb=4/5, and that A and b are in the same quadrants, evaluate cos(180-a).If you ask me.mow, my answer will.be 3/5 but my answer then was -3/5? Which of me is correct, thank for helping!
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  2. #2
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    Re: easy trigo question

    make use of the difference identity for cosine ...

    \cos(x-y) = \cos{x}\cos{y} + \sin{x}\sin{y}
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    Re: easy trigo question

    So cos(180-a) is -cosa?
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    Re: easy trigo question

    Quote Originally Posted by Tutu View Post
    So cos(180-a) is -cosa?
    did you use the given identity?
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