• To: mathgroup at smc.vnet.net
• Subject: [mg129646] Re: steady state concetration
• From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
• Date: Sat, 2 Feb 2013 01:17:07 -0500 (EST)
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```Hi,
i'm new in Mathematica and i've a project to do. i've to find at what time 90% of the steady state concetration of C is achieved. and soo far i have something like that, for 3 different k values:

With[{k = 0.3}, eqn1s = 0 == k (1/2 - z[t]) (1/3 - z[t])];
With[{k = 0.6}, eqn2s = 0 == k (1/2 - z[t]) (1/3 - z[t])];
With[{k = 0.9}, eqn3s = 0 == k (1/2 - z[t]) (1/3 - z[t])];
Evalute[{eqn1s, eqn2s, eqn3s}]
NSolve[Evaluate[{eqn1s, eqn2s, eqn3s}], z[t]]

and what i get in NSolve is supposedly be the steady state, but i don't know how to find this 90% and my profesor told me that i could try do this with DSolve, but something doesn't work. i hope that someone will be able to help me with this.

Thanks

Hi, Jeremy,

Your question is difficult to answer, since you show only a part of your problem. It seems that you should have
a differential equation describing the process. Otherwise, why z=z(t)?  In this case, however, in order to help you
one needs to see this equation.

Generally, if you have the ordinary differential equation, you may try DSolve, or,
if your equation is independent of parameters, try NDSolve. You will find  comprehensive examples at Menu/Help/DSolve and Menu/Help/NDSolve.

If you have equation depending upon one or two parameters, you may wrap your NDSolve statement by Manipulate or use ParametricNDSolve instead. I recommend that you try the both of them and then choose. Have a look at the corresponding places in the Menu/Help.

If your equation is a partial, Mathematica may only help in few special cases. Have a look into Menu/NDSolve: the heat and wave equation in one dimension in the end of the Section "Examples".
If you write down your differential equation, one may give a more precise advice. Nobody will solve it for you here, of course, but you may await to receive a useful hint of how to use Mathematica for that.

Have fun, Alexei

Alexei BOULBITCH, Dr., habil.
IEE S.A.
ZAE Weiergewan,
11, rue Edmond Reuter,
L-5326 Contern, LUXEMBOURG

Office phone :  +352-2454-2566
Office fax:       +352-2454-3566
mobile phone:  +49 151 52 40 66 44

e-mail: alexei.boulbitch at iee.lu<mailto:alexei.boulbitch at iee.lu>

```

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