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Math Help - Trigonometric Equation

  1. #1
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    Trigonometric Equation

    Hi there,

    I'm stuck on this question:

    Solve 2cos2x - 5cosx - 4 = 0 for 0≤ x < 2π.

    Thanks in advance
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  2. #2
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    Quote Originally Posted by HighlyFlammable View Post
    Hi there,

    I'm stuck on this question:

    Solve 2cos2x - 5cosx - 4 = 0 for 0≤ x < 2π.

    Thanks in advance
    Dear HighlyFlammable,

    Expand cos2x using, \cos 2x=2\cos^{2}x-1 and you will get a quadratic equation of cos x. Solve it and you can find a value for x.
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  3. #3
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    I did that and got 4cos^2x - 5cosx - 5 = 0 and I'm now unsure how to factorise it. Am I missing something obvious? I'm having a bit of a slow day.
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  4. #4
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    Quote Originally Posted by HighlyFlammable View Post
    I did that and got 4cos^2x - 5cosx - 5 = 0 and I'm now unsure how to factorise it. Am I missing something obvious? I'm having a bit of a slow day.
    Incorrect. It should be, 4\cos^{2}x-5\cos x-6=0
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