Re: problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg100330] Re: [mg100302] problem*From*: "David Park" <djmpark at comcast.net>*Date*: Mon, 1 Jun 2009 07:08:02 -0400 (EDT)*References*: <21606229.1243766725441.JavaMail.root@n11>

Sometimes, in attempting to use DSolve or Solve, it helps to avoid approximate numbers such as 0.5. So, I rewrote your differential equations as follows (but it didn't help any with DSolve): deqns = {y''[x] == -y[x]*((y[x])^2 + (z[x])^2)^(1/2), z''[x] == -z[x]*((y[x])^2 + (z[x])^2)^(1/2), y'[0] == 1, y[0] == 1, z'[0] == 1/2, z[0] == 1}; Clear[y, z]; dsols = First@NDSolve[deqns, {y, z}, {x, -100, 100}] {y[x_], z[x_]} = {y[x], z[x]} /. dsols ParametricPlot[{y[x], z[x]}, {x, -100, 100}, Frame -> True] David Park djmpark at comcast.net http://home.comcast.net/~djmpark/ From: parmida shabestary [mailto:dj_poni at yahoo.com] Hi I'm having trouble solving this set of equations: y''[x] =-y[x]*((y[x])^2 + (z[x])^2)^0.5 z''[x] =-z[x]*((y[x])^2 + (z[x])^2)^0.5 I couldn't write a DSolve order for it but here is the numerical code I wrote: NDSolve[{y''[x] == -y[x]*((y[x])^2 + (z[x])^2)^0.5, z''[x] == -z[x]*((y[x])^2 + (z[x])^2)^0.5, y'[0] == 0.5, z[0] == 1, y[0] == 1, z'[0] == 0.5}, {y, z}, {x, -100, 100}] I can plot this answer but what I really need is the parametric solution(a set of equations for z and y depending on x). Please help me find the answer. thanx parmida