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Math Help - coupled differential equation

  1. #1
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    coupled differential equation

    Hi my question is

    use any appropriate method to find the general solution to the coupled system of ODEs.
    <br />
\dot{y}-y+z=e^{-t}

    \dot{z}-2z=3e^{-t}

    i tried to differentiate first equation. but wasn't getting anywhere.
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  2. #2
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by cooltowns View Post
    Hi my question is

    use any appropriate method to find the general solution to the coupled system of ODEs.
    <br />
\dot{y}-y+z=e^{-t}

    \dot{z}-2z=3e^{-t}

    i tried to differentiate first equation. but wasn't getting anywhere.
    You can first solve the second equation for z(t) using the integrating factor technique, and then substitute your solution into the first equation.

    If you solve the second equation, you should get z(t)=-e^{-t}+Ce^{2t} (verify). So the first equation becomes \dot{y}-y=e^{-t}-(-e^{-t}+Ce^{2t})\implies \dot{y}-y=2e^{-t}+Ce^{2t}

    I leave it to you to finish this problem.
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  3. #3
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    Solve the homogeneous system of equations first (i.e. set the rhs to zero), and use the obtained solutions as the eigenfunctions of the system for the general solution. Make sense?
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  4. #4
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    Yes, but since this system is "partially uncoupled" that is the hard way. As Chris L T521 said, solve the second equation for z, then put that into the first equation.
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  5. #5
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    Is this possible to do it this way ?

    z=e^{-t}-\dot{y}+y hence \dot{z}=-e^{-t}-\ddot{y}+\dot{y}Using this information and substituting it into second equation

    we get (-e^{-t}-\ddot{y}+\dot{y})-2(e^{-t}-\dot{y}+y)=3e^{-t}
    <br />
-\ddot{y}+3\dot{y}-2y=0

    which gives us the polynomial  -\lambda^{2}+3\lambda-2=0which solves to (\lambda-1)(\lambda-2) which gives us y=Ae^{t}+Be^{2t}

    and then i can jst solve for z using intergrating factor method.
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