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Thread: Trig identities

  1. November 2nd 2008, 10:12 PM #1
    ssmh
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    Trig identities

    Cos(2x)=-(√2)/2

    Can anyone help me solve this?
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  2. November 2nd 2008, 11:07 PM #2
    11rdc11
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    Quote Originally Posted by ssmh View Post
    Cos(2x)=-(√2)/2

    Can anyone help me solve this?
    I kind of sleepy so hopefully I don't make a mistake lol

    \cos{(2x)}=-\frac{\sqrt{2}}{2}

    Ok so you know cos is negative in the 2nd and 3rd quadrant.

    Also \cos{x} = -\frac{\sqrt{2}}{2} at 135 degrees and 225

    Now just divide 135 and 225 by 2 and it will give you your answer

    67.5 + 360n

    112.5 + 360n

    n = 0,\pm 1, \pm 2,...
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