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Math Help - trig question

  1. #1
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    fratta terme forli Italy
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    trig question

    if i have cos theta = 0.4
    what are the values of

    cos (180-theta)
    cos (360-theta)

    i solve like this
    cos.4 inverse = 66.42
    therefore cos ( 180-66.42)
    i have cos *113.57 is this answer or must i make multiplication which gets me -.4 ??
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
    Joined
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    Quote Originally Posted by gregorio View Post
    if i have cos theta = 0.4
    what are the values of

    cos (180-theta)
    cos (360-theta)

    i solve like this
    cos.4 inverse = 66.42
    therefore cos ( 180-66.42)
    i have cos *113.57 is this answer or must i make multiplication which gets me -.4 ??
    there are several ways to approach this

    you can note that:

    \cos (180 - \theta ) = - \cos \theta

    and

    \cos (360 - \theta ) = \cos \theta

    OR

    you can do it the way you did, and you would get the same answer

    OR

    you can use the double angle formula:

    \cos (A - B) = \cos A \cos B + \sin A \sin B ........which, by the way, is how we know the first two formulas i gave you


    and, by the way, it is NOT cos * 113.57, it is cos(113.57). the cosine function is an argument, you cannot treat it like a term unto itself and multiply it with something
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