Re: 'Changing' parameters in an expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg60550] Re: [mg60528] 'Changing' parameters in an expression*From*: <bsyehuda at gmail.com>*Date*: Tue, 20 Sep 2005 05:19:04 -0400 (EDT)*References*: <200509190845.EAA23546@smc.vnet.net>*Reply-to*: bsyehuda at gmail.com*Sender*: owner-wri-mathgroup at wolfram.com

Hi, First, the expression eqn[[2]]/.a->1/2 equals to (-1/2+x[t])x[t] x[t] is still unknown (you didn't solve the differential equation yet), so what are you trying to plot? Plot returns an answer that it cannot get a number for each value of t in the range required for the plot. You didn't define initial conditions as well. For the specific case of your differential equation you can use Solve Solve[eqn,x[t],t] and get (in Inputform format) Out[10]= {{x[t] -> (a*E^(a*C[1]))/(-E^(a*t) + E^(a*C[1]))}} where C[1] need to be determined by a specific initial condition. When you determine this initial condition wou will be able to plot the results yehuda On 9/19/05, Matt <anonmous69 at netscape.net> wrote: > > Hello, > What follows (at the end) worked for me, however, I'm wondering if > it's the 'right' approach or not. What I mean by 'right' is this: my > eventual approach worked, but there may be another approach that is > more efficient and demonstrates a more wide ranging principal in using > Mathematica, and hence would be applicable in many situations. > > I started off with this: > eqn = x'[t] == x[t](x[t] - a); > > (I am using eqn as the first argument to DSolve, which is why it > appears as it does.) > > I wanted to plot the rhs of eqn with 'a' having a value of 1/2, so > first I tried this: > Plot[eqn[[2]] /. a->1/2, {t, 0, 6}]; > > That didn't work as I got these types of errors: > Plot::plnr : eqn[[2]] /. a -> 1/2 is not a machine-size real number at > t = .... > > over and over again. > > I tried a few other rules with essentially the same results and then I > tried this: > Plot[eqn[[2]] /. {a->1/2, x[t]->x}, {x, 0, 6}]; > > Which worked for me. I'm sure it has something to do with what I'm > asking Plot to work with. It just seems strange to me that DSolve will > work with eqn, but Plot does not. > > Thanks for any help, > > Matt > >

**References**:**'Changing' parameters in an expression***From:*"Matt" <anonmous69@netscape.net>