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Math Help - Differential Equation help

  1. #1
    Super Member craig's Avatar
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    Differential Equation help

    I have the DE, \frac{dP}{dt} = B_0e^{-at}P, which I've solved to give P = P_0 e^{\frac{B_0}{a}\left(1 - e^{-at}\right)}.

    I'm also given that B(t) = B_0e^{-at} and that P(0) = P_0.

    Q) Suppose that at time t = 0 there are P_0 = 10^6 cells and that P(t) is increasing at the rate of 3 \times 10^5 cells per month. After 6 months the tumor has doubled (size and number of cells). Find a.

    A)
    We can say that P = 10^6 e^{\frac{B_0}{a}\left(1 - e^{-at}\right)}.

    If the tumor has doubled in 6 months, then does this mean that P(6) = 2P(0)

    10^6 e^{\frac{B_0}{a}\left(1 - e^{-a6}\right)} = 2(10^6 e^{\frac{B_0}{a}\left(1 - e^{0}\right)}), therefore e^{\frac{B_0}{a}\left(1 - e^{-6a}\right)} = 10^6(1)

    Also if P(t) is increasing at a rate of 3 \times 10^5, I presume that \frac{dP}{dt}=3 \times 10^6, or is it B(t)= 3 \times 10^6?

    If someone could give me a gentle push in the right direction it would be much appreciated

    Thanks a lot

    Craig
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  2. #2
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    From your information, I can say:

    \ln(2x10^6)=-\frac{B_0}{a}e^{-6a}+c

    and you can easily find c from the initial population at t=0. But the initial derivative means 3x10^5=-\frac{B_0}{a}+c. Now, taking this expression and substituting it into the first expression, can you not find a?
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  3. #3
    Super Member craig's Avatar
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    Quote Originally Posted by shawsend View Post
    From your information, I can say:

    \ln(2x10^6)=-\frac{B_0}{a}e^{-6a}+c

    and you can easily find c from the initial population at t=0. But the initial derivative means 3x10^5=-\frac{B_0}{a}+c. Now, taking this expression and substituting it into the first expression, can you not find a?
    Hi Shawsend thanks for the reply. If you don't mind could you explain where you got your values from, having a little difficultly in following along.

    Also, do you mean \ln(2x10^6)=-\frac{B_0}{a}e^{-6a}+c or \ln(2 \times 10^6)=-\frac{B_0}{a}e^{-6a}+c?

    Thanks again

    Craig
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  4. #4
    Super Member craig's Avatar
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    Here's what I've come up with so far.

    Using the fact that P(6) = 2P(0),

    10^6 e^{\frac{B_0}{a}\left(1 - e^{-6a}\right)} = 2 \cdot10^6 e^{\frac{B_0}{a}\left(1 - e^{0}\right)}

    e^{\frac{B_0}{a}\left(1 - e^{-6a}\right)} = 10^6

    ln{(10^6)} = \frac{B_0}{a}\left(1 - e^{-6a}\right)

    Having trouble finding a second equation in terms of just a and B_0.

    We also know that when t=0, P(t) is increasing at 3 \times 10^5 cells per month.

    Using the expression for \frac{dP}{dt}, and using t=0, we get the following:

    3 \times 10^5 = B_0 \cdot P.

    Not sure what to do from here, how to eliminate the P?

    Thanks again for anyone reading through this
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  5. #5
    Super Member craig's Avatar
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    Sorry I'm just carrying on with myself here aren't I, every time I post I seem to get a new idea how to tackle this problem.

    From what I've said earlier, I've got the two following equations:

    ln{(10^6)} = \frac{B_0}{a}\left(1 - e^{-6a}\right)

    B_0 \cdot P = 3 \times 10^5

    The second equation is true when t=0, so what I thought is to use the value of P when t=0, ie P_0.

    This results in B_0 10^6 = 3 \times 10^5, B_0 = 0.3

    Now if I put this value into my first equation, this gives me:

    ln{(10^6)} = \frac{0.3}{a}\left(1 - e^{-6a}\right)

    Using Maple I've solved this to give me a value of a=-0.5430780576.

    However I know that both a and B_0 are positive and real constants, so obviously something has gone wrong??
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  6. #6
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    Hi. Craig. Sorry. I made a mistake up there. I get:

    \ln(2)=\frac{3}{10a}\left(1-e^{-6a}\right)

    That's a very interesting equation to solve for a explicitly in terms of the Lambert W function. We can show:

    a=\frac{9+5 \text{Log}[2] \text{ProductLog}\left[-\frac{9 e^{-\frac{9}{5 \text{Log}[2]}}}{5 \text{Log}[2]}\right]}{30 \text{Log}[2]}\approx 0.3915

    Here's how I did it:

    We have: k=\frac{b}{a}(1-e^{-6a})

    ake^{6a}-be^{6a}=-b

    (6ka-6b)e^{6a}=-6b

    (6a-\frac{6b}{k})e^{6a}=-\frac{6b}{k}

    (6a-\frac{6b}{k})e^{6a-\frac{6b}{k}}=-\frac{6b}{k}e^{-6b/k}

    6a-\frac{6b}{k}=\text{W}\left(-6b/k e^{-6b/k}\right)

    a=\frac{b}{k}+\frac{1}{6}\text{W}\left(-\frac{6b}{k}e^{-6b/k}\right)
    Last edited by shawsend; December 7th 2009 at 09:09 AM.
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  7. #7
    Super Member craig's Avatar
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    Hi thanks for the quick reply. When I tried to solve through Maple there was something about the Lambert W function.

    Quote Originally Posted by shawsend View Post
    \ln(2)=\frac{3}{10a}\left(1-e^{-6a}\right)
    I understand the RHS of the equation, using the value of B_0 = 0.3. But how did you get \ln(2) and I got \ln(10^6)?

    Thanks again
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  8. #8
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    We have the two equations:

    \ln(10^6)=-\frac{3}{10a}+c\rightarrow c=\ln(10^6)+\frac{3}{10 a}

    and:

    \ln(2\cdot 10^6)=-\frac{3}{10 a}e^{-6a}+c

    then:

    \ln(2)=\frac{3}{10 a}(1-e^{-6a})
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  9. #9
    Super Member craig's Avatar
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    Sorry about all these questions, you have the patience of a saint

    I understand how you got from the two equations to give the answer for a. I don't get how you formed these two equations though?

    You have a 2 \times 10^6, but didn't these cancel out in this equation, 10^6 e^{\frac{B_0}{a}\left(1 - e^{-6a}\right)} = 2 \cdot10^6 e^{\frac{B_0}{a}\left(1 - e^{0}\right)}?

    Also I'm not sure where the + c has come from either? Obviously we need a constant of integration but didn't all this get dealt with when I first solved the differential equation in the first place to get an equation for P?

    Sorry for the confusion, clearly something completely obvious I've failed too see

    All I can say is thank you so much for the help, you've been an absolute life saver.
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  10. #10
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    We start with:

    P'=B_0 e^{-at}P

    the solution I'll leave in log form is:

    \ln(P)=-\frac{B_0}{a} e^{-at}+c

    Now, using the three given conditions:

    At t=0, the derivative information tells us: 3\cdot 10^5=B_0 10^6\Rightarrow B_0=\frac{3}{10}

    At t=0, the initial conditions tells us: \ln(10^6)=-\frac{3}{10 a}+c

    And at t=6, we have:

    \ln(2\cdot 10^6)=-\frac{3}{10 a} e^{-6t}+c
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  11. #11
    Super Member craig's Avatar
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    Quote Originally Posted by shawsend View Post
    We start with:

    P'=B_0 e^{-at}P

    the solution I'll leave in log form is:

    \ln(P)=-\frac{B_0}{a} e^{-at}+c

    Now, using the three given conditions:

    At t=0, the derivative information tells us: 3\cdot 10^5=B_0 10^6\Rightarrow B_0=\frac{3}{10}

    At t=0, the initial conditions tells us: \ln(10^6)=-\frac{3}{10 a}+c

    And at t=6, we have:

    \ln(2\cdot 10^6)=-\frac{3}{10 a} e^{-6t}+c
    Thankyou, that's really helped explain it.
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