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Math Help - solving trig equations

  1. #1
    Newbie Tom G's Avatar
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    solving trig equations

    Can someone please check that I've got the correct answers as they don't appear to fit.

    Q: cos2x = -(√3)/2 -180<=x<=180

    I've got 2x=210, 330, 570, 690
    x=105, 165, 285, 345

    We're currently learning about using CAST diagrams to solve trigonometric equations, so if any anyone can explain in terms of CAST diagrams I'd be grateful.
    Last edited by Tom G; February 7th 2007 at 11:44 AM. Reason: Clarify what cos2x equals.
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  2. #2
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    Hello, Tom!

    You picked some incorrect angles . . .


    cos(2x) = -(√3)/2, . -180 < x < 180

    Cosine is negative in Quadrants 2 and 3.

    . . 2x .= .-210, -150, 150, 210


    Therefore: .x .= .-105, -75, 75, 105

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  3. #3
    Grand Panjandrum
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    Quote Originally Posted by Tom G View Post
    Can someone please check that I've got the correct answers as they don't appear to fit.

    Q: cos2x = -(√3)/2 -180<=x<=180

    I've got 2x=210, 330, 570, 690
    x=105, 165, 285, 345

    We're currently learning about using CAST diagrams to solve trigonometric equations, so if any anyone can explain in terms of CAST diagrams I'd be grateful.
    Your answers other than the first are not in the specified range for x.

    My calculations give: x=75, 105, -75, -105 degrees. Which check out
    numericaly.

    RonL
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