"statira" wrote in message <jbg6pc$bu0$1@newscl01ah.mathworks.com>...
> "statira" wrote in message <jbg3i8$33b$1@newscl01ah.mathworks.com>...
> > My question is simple. How can I solve the system of differential equations like these?
> >
> > y"=f(y,y',x,x',x")
> > x"=g(x,x',y,y',y")
> >
> > Can Ode45i solve equations like these? Haw?
>
> My equations are:
> (y'+1)*x''x'*y''+x+y2=0
> y''*x+(3y)*x''+4*y'+8=0
      
If you let z(1) = x, z(2) = x', z(3) = y, and z(4) = y', just use the four implicit equations for ode45i:
z(4)'  f(z(3),z(4),z(1),z(2),z(2)') = 0
z(2)'  g(z(1),z(2),z(3),z(4),z(4)') = 0
z(1)'  z(2) = 0
z(3)'  z(4) = 0
However, I don't see why you need to bother with ode45i when it is so easy to explicitly solve for x'' and y'' in terms of the lower derivatives as shown in your second post. It is after all linear in x'' and y''. Then you could use ode45 instead.
Roger Stafford
