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Solving nonlinear differential equations

• To: mathgroup at christensen.cybernetics.net
• Subject: [mg896] Solving nonlinear differential equations
• From: mdc111 at psu.edu (Marci Mouer)
• Date: Fri, 28 Apr 1995 00:53:57 -0400

I have been trying to find the symbolic solutions to a set 
of 6 differential equations using DSolve.  The 6 equations 
are listed below:

  x1'[t] == x2[t]
  x2'[t] == g*((R*Cos[x5[t]])-x1[t])/L
  x3'[t] == x4[t]
  x4'[t] == g*((R*Sin[x5[t]])-x3[t])/L
  x5'[t] == x6[t]
  x6'[t] == u+(m*g*R*((x3[t]*Cos[x5[t]])-(x1[t]*Sin[x5[t]]))/(J1*L))

When I try to get the solution, Mathematica simply restates
my problem.  From some quick experiments, I believe that the
source of the problem is from taking Sin[x5[t]] and 
Cos[x5[t]] instead of the independent variable t.

Is there some way to get around this?  (In case it matters,
I am running a version 2.1.)  Any help would be appreciated.  

Marci Mouer