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

What do you do if your exact equation isn't exact? and they give u an Integrating F

  1. Feb 1, 2006 #1
    Hello everyone I understand how to solve exact equations, but what happens when they arnt' exact? I'm confused on what i'm suppose to do! Does anyone feel like explaning hte process to me, if given an integrating factor/> or give me a website? Here is my problem:
    Check that the equation below is not exact but becomes exact when multiplied by the integrating factor.
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/dd/9b5141797d9106d32db40a0eac2dc81.png [Broken]
    Integrating Factor:
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/26/0eab7b29675ba3a7a3db0668549eb81.png [Broken]
    Solve the differential equation.
    You can define the solution curve implicitly by a function in the form
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/6b/90f9e4bd0e95299118e1c30fd8981d1.png [Broken]
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/8d/8616ca1dd6ce230911485b98a57fa31.png [Broken] ?

    Thank you!
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 1, 2006 #2


    User Avatar
    Science Advisor

    Do you understand what an "integrating factor" is?

    If a differential equation is not exact then there always exists some function f(x,y) so that multiplying the equation by it makes it exact. Unfortunately, it's most often very difficult (if not impossible) to find that integrating factor!

    In this case your equation is x2y3dx+ x(1+ y2)dy= 0. Yes, that is NOT exact because (x2y3)y= 3x2y2 which is not the same as (x(1+ y2))x= 1+y2.

    Fortunately, you were told that [itex]\frac{1}{xy^3}[/itex] is an integrating factor. That means that multiplying the equation by that:
    (1/xy2){x2y3dx+ x(1+y2)dy
    = ydx+ (1+y2)y2)dy= 0.

    Is that exact? It certainly should be! If it is exact can you solve it?
  4. Feb 2, 2006 #3
    OOoo!!! Awesome, thanks alot IVEY as always! It worked great! I got:
    http://cwcsrv11.cwc.psu.edu/webwork2_files/tmp/equations/3c/d002938fe55dd46091d4edaff05c161.png [Broken]
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook