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Math Help - cos-1(0) = 90, so cos 90 = 0 but...

  1. #1
    s3a
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    cos-1(0) = 90, so cos 90 = 0 but...

    (I am talking in degrees)
    cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

    I know that cos (theta) = cos (-theta) by the way.
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  2. #2
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    Quote Originally Posted by s3a View Post
    (I am talking in degrees)
    cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

    I know that cos (theta) = cos (-theta) by the way.
    Well technically there's an infinite number of solutions, so it just gives the one nearest to 0.
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  3. #3
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    Quote Originally Posted by s3a View Post
    (I am talking in degrees)
    cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

    I know that cos (theta) = cos (-theta) by the way.
    Read this: Trigonometry: Inverse Cosine and Inverse Sine - CliffsNotes
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