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Solving First Order Differential Equation

  1. Mar 4, 2014 #1
    1. The problem statement, all variables and given/known data

    I have been trying to solve this equation but keep coming to the same solution, which according to my book is not the correct one. Is anyone able to point out what I am doing wrong?

    [tex]\frac{dy}{dt}-\frac{1}{2}y=2cos(t)[/tex]




    3. The attempt at a solution

    To solve, use the integrating factor [tex]e^{\int\frac{1}{2}dt}[/tex]; Integrating factor=[tex]e^{-\frac{t}{2}}[/tex]

    [tex] ye^{-\frac{t}{2}} = 2\int e^{-\frac{t}{2}}cos(t)dt[/tex]




    Integrating the RHS by parts:

    [tex] = 2\left[e^{-\frac{t}{2}}sin(t)+\frac{1}{2}\int sin(t)e^{-\frac{t}{2}}dt\right][/tex]

    [tex] = 2\left[e^{-\frac{t}{2}}sin(t)+\frac{1}{2}\left[e^{-\frac{t}{2}}\cdot(-cos(t)-\frac{1}{2}\int cos(t)e^{-\frac{t}{2}}dt\right]\right][/tex]




    And using a reduction formula:

    [tex] = 2\left[e^{-\frac{t}{2}}sin(t)-\frac{e^{-\frac{t}{2}}cos(t)}{2}-\frac{1}{4}I\right][/tex]

    [tex] I =2e^{-\frac{t}{2}}sin(t)-e^{-\frac{t}{2}}cos(t)-\frac{1}{2}I[/tex]

    [tex]\frac{3}{2}I=2e^{-\frac{t}{2}}sin(t)-e^{-\frac{t}{2}}cos(t)[/tex]

    [tex] I = \frac{2}{3}(2e^{-\frac{t}{2}}sin(t)-e^{-\frac{t}{2}}cos(t))[/tex]

    [tex]ye^{-\frac{t}{2}}=\frac{4e^{-\frac{t}{2}}sin(t)-e^{-\frac{t}{2}}cos(t)}{3}[/tex]

    [tex] y = \frac{4 sin(t)-cos(t)}{3}+ce^{\frac{t}{2}}[/tex]




    After all this, the book gives a solution of [tex]y=\frac{4}{5}(2sin(t)-cos(t))+ce^{\frac{t}{2}}[/tex]
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 4, 2014 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    hi jellicorse! welcome to pf! :smile:

    good method, but i think you've used ye-t/2 = 2I in one place and = I in another place :wink:
     
  4. Mar 4, 2014 #3
    Ah, thanks tiny-tim... I will look into that!
     
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