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Homework Help: First order diff eq

  1. Aug 10, 2010 #1
    1. The problem statement, all variables and given/known data

    Salt water pours into a 10 liter tank at a rate of 4 l/min. Its concentration is 2 g/l. The brine in the tank is well mixed and it drains out at a rate of 4 l/min. Call y the grams of salt in the tank at time t. The tank is initially full of fresh water. Solve the differential equation that models the salt in the tank.

    2. Relevant equations

    dy/dx = rate in - rate out
    y(t)= grams of salt (y) at time (t)

    3. The attempt at a solution

    I worked the problem and got the differential equation y'=8-(y/10)4 now I don't know how to solve that to get a particular solution assuming the initial condition y(0)=0
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 10, 2010 #2
    Your equation is fine. Now you can separate the variables (y and t) and integrate both sides.

    [tex] \frac{dy}{dt} = 8 - \frac{y}{2.5} [/tex]

    When you are ready to integrate, this will look like:

    something with only ys dy = something with only ts dt

    Then use the initial condition to solve for your constant of integration.
  4. Aug 10, 2010 #3


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    Homework Helper

    Actually, I think that DE will require an integrating factor.

    EDIT: Nevermind...constants are making me overkill it.
    Last edited: Aug 10, 2010
  5. Aug 10, 2010 #4
    ok so here's what i got so far:

    integrate to get
    (-1/4)ln(80-4y)=t/10 + C

    what do you think?
  6. Aug 10, 2010 #5
    That looks okay to me, now solve for y.

    (you can do this with an integrating factor, too, but I don't see that's it's necessary).
  7. Aug 10, 2010 #6
    Ok if what I have so far is correct solving for y gets me:

    y(t)=20 + e^(-2t/5)C

    then assuming the inital condition of y(0)=0 I get C= -20

    so the final answer would be y(t)=20-20e^(-2t/5)

    can someone verify this is correct?
  8. Aug 10, 2010 #7
    You can verify it easily yourself, right? Take the derivative of y, plug in to the original equation, make sure it works out.

    Yes, it does. But you should still do it yourself! This is one of the best things about differential equations. It's usually really easy to check your work, because taking derivatives of functions is easy.
  9. Aug 10, 2010 #8


    Staff: Mentor

    You can. Check that
    1) y(0) = 0, using your solution.
    2) y' = 8 - 2y/5, using your solution.

    That's all you need to do to verify your solution.
  10. Aug 10, 2010 #9
    perfect! I know I did check it myself but I am very poor in math skills I like to have another set of eyes thanks so much for your quick responses
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