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    Find the value for A for which Axcos3x is a particular integral of the differential equation
    d²y/dx² +9y=-12sin3x

    I've tried subbing in y=Axcos3x into the LHS but that gives me sin3x and cos3x terms both with A in them so I can't compare coefficients...
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    What do you get for the following:

     \dfrac{d^2}{dx^2} (A  x \cos(3x) )

    (Original post by bobbricks)
    Find the value for A for which Axcos3x is a particular integral of the differential equation
    d²y/dx² +9y=-12sin3x

    I've tried subbing in y=Axcos3x into the LHS but that gives me sin3x and cos3x terms both with A in them so I can't compare coefficients...
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    (Original post by bobbricks)
    Find the value for A for which Axcos3x is a particular integral of the differential equation
    d²y/dx² +9y=-12sin3x

    I've tried subbing in y=Axcos3x into the LHS but that gives me sin3x and cos3x terms both with A in them so I can't compare coefficients...
    Try again. I think you have made a mistake with the differentiation or substitution.
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    I get d²y/dx²=-6Asin3x-9Axcos3x ..is that correct?

    EDIT: got A=2
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    Looks good.

    (Original post by bobbricks)
    I get d²y/dx²=-6Asin3x-9Axcos3x ..is that correct?

    EDIT: got A=2
 
 
 
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