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

Differential Equation

  1. Mar 31, 2008 #1
    The solution of the initial value problem, dy/dx = y-3. y(0) = 4, what does y=?

    dy/y-3 = dx
    1/y-3 dy = dx
    ln(y-3) = ? + C

    I have no idea how to do this one, can anybody please help me out? I'm having a hard time seeing how to separate the y and the -3 or if they are separable at all.
  2. jcsd
  3. Mar 31, 2008 #2
    well you started it right:


    Now to get our answer we integrate both parts

    [tex]\int\frac{dy}{y-3}=\int dx=>ln(y-3)=x+C[/tex]

    Now do you know how to isolate y by itself. Hint: remember that ln and the exponential function with base "e" are inverses. So, exponentiate both sides, and try to go as further as you can. If you're still stuck, ask again.
  4. Mar 31, 2008 #3
    I do. thanks stupidmath. I guess it was the "invisible" x that got me.

    y-3 = Ce^x
    y = Ce^x + 3
  5. Mar 31, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Now apply your initial conditons to find C.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Differential Equation
  1. Differential Equations (Replies: 6)

  2. Differential Equation (Replies: 12)