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

Homework Help: Ok one more ODE: u'=(x(e^x))-u

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data

    "Solve the following ODE's:"

    2. Relevant equations


    3. The attempt at a solution


    u'=xe^x => u=(xe^x)/ln(xe)
    u'=-u => u=e^-x

    I'm unsure how to combine these two answers.
  2. jcsd
  3. Oct 13, 2008 #2


    User Avatar
    Homework Helper

    I don't follow what you are trying to do here. Where did ln come from? And how did you get u' = -u ? And what is "combine" suppose to mean?

    The original DE is itself already in the form of a easily recognisable type of DE.
  4. Oct 13, 2008 #3
    What I was trying to do was separate the xe^x and the (-u) and deal with them independently and then combine them after words.

    Int(A+B) is the same as Int(A)+Int(B) right?

    But if their is a better way to do that please let me know which method to try, my diffy eq is very rusty.
  5. Oct 13, 2008 #4


    User Avatar
    Homework Helper

    You can't just integrate it like that. You have to make sure the variables are properly separated before you can apply the separation of variables technique. In its present form, the DE isn't separable. A large factor in solving DE's analytically (by hand rather than numerical methods) is learning how to recognise a certain type of DE when you see one. What have you learnt about how to identify this type of DE?
  6. Oct 13, 2008 #5
    Well I've been through diffy eq, but that was about 4 years ago. The review I've gotten for this class is:
    1st or 2nd order ODE
    Linear or Non Linear
    Homogeneous or Non Homogeneous

    We've reviewed several methods for solving these:
    Seperable Equations
    Method of Integration by Factors
    Method of Variation of Parameters

    I suppose the biggest problem is I've lost the small grasp I had on diffy eq. I just need to get back in practice. This is our first homework assignment, I know there is more to come.

    Thanks for your help everyone!
  7. Oct 13, 2008 #6


    User Avatar
    Homework Helper

    I can tell you for one thing the equation isn't separable, so you have to try out another approach. The method you should use is already outline in those which you said your class reviewed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook