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MathGroup Archive 2006

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Re: Differential equation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69493] Re: Differential equation
  • From: dimmechan at yahoo.com
  • Date: Thu, 14 Sep 2006 06:55:32 -0400 (EDT)
  • References: <ee8gca$imk$1@smc.vnet.net>

***You have one differentai equation for two dependent variables.
***You need one more equation.

diff=x[t]-A*D[y[t],t]==0;

DSolve[diff,{x[t],y[t]},t]
DSolve::underdet: The system has more dependent variables than
equations, so is underdetermined. Moreâ?¦
DSolve[x[t] - A*Derivative[1][y][t] == 0, {x[t], y[t]}, t]

***But e.g.

diff1=x[t]-A*D[y[t],t]==0;
diff2=D[x[t],t]+y[t]==2;

DSolve[{diff1,diff2},{x[t],y[t]},t]
{{y[t] -> C[1]*Cos[t/Sqrt[A]] + 2*Cos[t/Sqrt[A]]^2 +
(C[2]*Sin[t/Sqrt[A]])/Sqrt[A] + 2*Sin[t/Sqrt[A]]^2, x[t] ->
C[2]*Cos[t/Sqrt[A]] - Sqrt[A]*C[1]*Sin[t/Sqrt[A]]}}

***Cheers
Dimitris
Î?/Î? Mike έγÏ?αÏ?ε:
> Dear all,
>
> I would kindly ask for an assistance please.
> I do not know how to solve this differential equation :
>
> x(t)-A*dy/dt=0
>
> Where,
> A - constant
> x - variable as a function of time (t)
> y - variable as a function of time (t)
> t - time
> 
> 
> Thanks Mike77


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