Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help solving this differential equation

  1. Mar 12, 2013 #1
    I've separated the variables of this differential equation and end up with
    dx/((a-x)^(1/2)*(b-c(x-d)^3/2)). I've tried finding the integral of this with non-trig substitution methods but cannot solve it. Any help would be appreciated.
  2. jcsd
  3. Mar 12, 2013 #2


    User Avatar
    Homework Helper

    I don't see an equation here. (Where's the equals sign?) Start by writing the original problem and show us what work you have.
  4. Mar 12, 2013 #3
    The original diff eq is

    dx/dy = b(a-x)^1/2 - c(a-x)^1/2 * (x-d)^3/2

    Separating variables results in my original posted equation

    dy = dx/((a-x)^1/2 * (b-c(x-d)^3/2))

    I have tried the substitution, u = (a-x)^1/2, x = a-u^2, dx = -2udu. Which results in

    dy = -2dx/(b-c((-u^2+a)-d)^3/2)

    Any further non-trig substitutions does not help to simplify. I believe a trig substitution is required but I have little experience with trig subs. I have already put any many hours looking for a solution and would like to know if a trig sub could be used to solve this. Thanks
  5. Mar 12, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I don't think there's any point in manipulating the ODE. You have reduced it to an integral, so work with that. Your substitutions so far look good, but I believe you can simplify it to ##\frac{du}{A-(1-u^2)^{\frac32}}##. Substituting u = sin(θ) and expanding with partial fractions can get you to a sum of terms like ##\frac{d\theta}{W-cos(\theta)}##, but I don't know where to go from there.
  6. Mar 14, 2013 #5
    I'm not seeing anywhere to go after that either. But thanks for the input.
  7. Mar 14, 2013 #6

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    The integral is VERY complicated. It involves a lengthy formula that uses the roots of a 6th degree polynomial whose coefficients are functions of a, b, c and d. I did the integral in Maple, but if you do not have access to Maple you could try to submit it to Mathematica. Wolfram Alpha failed to find the integral.
  8. Mar 14, 2013 #7
    Glad to hear that there is a solution. Since this thread is no longer in the schoolwork forum, would you be more forthcoming with the solution.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook