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...