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Math Help - differential equation help

  1. #1
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    differential equation help

    OK. I'm working on a homework problem in Diffy Equ. and I'm stuck halfway through. It is a homogenius equation w/ constant coefficients.

    y'''-8y''-y'+8y=0

    y(0)=-7, y'(0)=7, y''(0)=-7

    I've gotten as far as:

    y= C1e^x+C2e^-x+C3e^-8x
    y'= C1e^x+c2e^-x-8C3e^-8x
    y''= C1e^x+C2e^-x+64C3e^-8x

    c1+c2+c3=-7
    c1+c2-8c3=7
    c1+c2+64c3=-7

    I'm getting stuck on solving for c1 c2 and c3 which I guess is more algebra than anything. If someone could walk me this it would be great. I was using substitution. After I get this I can do the rest.

    Thanks
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  2. #2
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    Quote Originally Posted by tinkerbell03815 View Post
    OK. I'm working on a homework problem in Diffy Equ. and I'm stuck halfway through. It is a homogenius equation w/ constant coefficients.

    y'''-8y''-y'+8y=0

    y(0)=-7, y'(0)=7, y''(0)=-7

    I've gotten as far as:

    y= C1e^x+C2e^-x+C3e^-8x
    y'= C1e^x+c2e^-x-8C3e^-8x
    y''= C1e^x+C2e^-x+64C3e^-8x

    c1+c2+c3=-7
    c1+c2-8c3=7
    c1+c2+64c3=-7

    I'm getting stuck on solving for c1 c2 and c3 which I guess is more algebra than anything. If someone could walk me this it would be great. I was using substitution. After I get this I can do the rest.

    Thanks
    Check your derivatives. Especially the sign on the second term.

    -Dan
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  3. #3
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    Thanks, I got it. I knew I was making some kind of stupid mistake
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