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ODE solution with Mathematica 8

  1. Feb 29, 2012 #1
    I'm having issues solving a 1st order ODE.
    Here what happens:

    NDSolve[{y'[x]/y[x] == k *Pi* r^2* Sqrt[1 + y'[x]^2], y[0] == 136/10}, y[x], {x, 0, 30}]
    NDSolve::ndnum: Encountered non-numerical value for a derivative at x == 0.

    Changing the x range is no help... I get the same message for any initial point...

    k and r are given parameters.

    Any idea what's wrong in my code?

    Thanks
     
  2. jcsd
  3. Feb 29, 2012 #2

    phyzguy

    User Avatar
    Science Advisor

    Do you specify the values of k and r before executing the NDSolve? NDSolve is a numerical solution, so it can only work with numbers, not variables.

    Alternatively, you can try using DSolve. Mathematica was able to find an analytic solution.
     
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