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Math Help - sin identities

  1. #1
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    sin identities

    Solve equation for values between o and 2 pi

    2cos2A + 1 + sinA = 0
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  2. #2
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    use the identity:
    \cos{2\theta} = 1 - 2\sin^{2}{\theta}

    Then try to rearrange the equation as necessary for factoring. If need be, use quadratic formula:
    x = \frac{-b\pm \sqrt{b^2 - 4ac}}{2a}

    EDIT: You may also use  \cos^2{\theta} + \sin^2{\theta} = 1
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  3. #3
    Senior Member nikhil's Avatar
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    Lightbulb Check this out

    Quote Originally Posted by gracey View Post
    Solve equation for values between o and 2 pi

    2cos2A + 1 + sinA = 0
    cos2A=1-2(sinA)^2 therfor
    the equation becomes
    2(1-2(sinA)^2)+1+sinA=0
    2-4(sinA)^2+1+sinA=0
    4(sinA)^2-sinA-3=0
    this is a quadratic equation.solving this we get
    sinA=1 or sinA=-3/4
    1)sinA=1
    A=pi/2
    2)sinA=-3/4
    let sinB=3/4(NOT -3/4)
    SO B=arcsin3/4
    so
    A=pi+B,2pi-B
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