       Re: Plot using DSolve HELP

• To: mathgroup at smc.vnet.net
• Subject: [mg104419] Re: Plot using DSolve HELP
• From: "Kevin J. McCann" <Kevin.McCann at umbc.edu>
• Date: Sat, 31 Oct 2009 01:51:03 -0500 (EST)
• References: <hce3uu\$r15\$1@smc.vnet.net>

```Your differential equation, eq, is not set equal to anything. perhaps
you meant

eq = i * v[t]/r +c v'[t] == 0

With i = 0, then v'=0 and the answer is just a constant -70.

Kevin

Becky wrote:
> I have a problem with what I am showing below.
>
> Step one is my equation.
> eq = i * v[t]/r +c v'[t]
>
> Step two I want to solve it, using DSolve, and put my values in as shown.
> Please note, that "i= 0", and my IC is v== -70.
>
> sol=v[t]/.DSolve[{eq/.{c->3, i->0, r->2.5} ,v==-70}, v[t], t]
>  {-70 *-0.133333 t}
> The results are shown above.
>
> Thus next step is to plot it.
> p1=Plot[sol,{t,-20,140}]
>
> My problem is that I get a line from -70 on the vertical axis going in a
> curve up to the horizontal axis out to 140.  The vertical axis is "v" and
> the horizontal is "t".
>
> What am I doing wrong?
>
> Thanks
> Jake
>
>

```

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