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

Why not separable (basic question)?

  1. Oct 31, 2009 #1
    In Example 1.5 of Differential Equations Demystified, the equation is y' +2xy = x, and the author claims this is not separable. Now, what am I missing. I try the following.

    1) dy/dx = x - 2xy
    2) dy/dx = x(1-2y)
    3) dy/(1-2y) = xdx

    I guess one of those is invalid but I just cannot identify which. Any help appreciated, thanks. This is part of an effort to brush up on ODE since it's been years since I took it and now I'm going for a Master's in Applied Math, need lots of help of this kind. Thanks
  2. jcsd
  3. Oct 31, 2009 #2


    User Avatar
    Gold Member

    Hmm, are you sure that's the equation they gave? I don't see what's wrong with that.
  4. Oct 31, 2009 #3
    Yes, that's definitely the equation, and the author says
    "This equation is plainly not separable (try and convince yourself that this is so)."
    Last edited: Nov 1, 2009
  5. Oct 31, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    You just separated it, disproving the author's claim.
  6. Nov 1, 2009 #5
    Author wrote me back saying that it was a mistake, so that is in fact a separable equation.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why not separable (basic question)?