       Re: problems with DSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg99436] Re: problems with DSolve
• From: schochet123 <schochet123 at gmail.com>
• Date: Tue, 5 May 2009 06:03:19 -0400 (EDT)
• References: <gtnk4s\$3md\$1@smc.vnet.net>

```The problem is not with DSolve but with Plot, which evaluates its
first argument in non-standard fashion. The remedy is to enclose that
argument inside the command Evaluate.

For example,

Manipulate[Plot[Evaluate[(x[t] /. DSolve[{x'[t] == y[t], y'[t] == -=
x
[t], x == a,
y == 2}, {x, y}, t])], {t, 0, 5}],{a,1,2}]

Steve

On May 4, 11:45 pm, szymon.st... at gmail.com wrote:
> hello,
>
> i am new to mathematica, therefore sorry if my question is trivial. I
> would like to understand why the following line does not work:
>
> Plot[(x[t] /. DSolve[{x'[t] == y[t], y'[t] == -x[t], x == =
1, y
> == 2}, {x, y}, t]), {t, 0, 5}]
>
> whereas after splitting into two lines it works:
> sol = DSolve[{x'[t] == y[t], y'[t] == -x[t], x == 1, y[0=
] == 2},
> {x, y}, t]
> Plot[(x[t] /. sol), {t, 0, 5}]
>
> i would like to use Manipulate to play with boundary conditions, so
> the first notation would be highly useful.
>
> thanks a lot for any feedback.

```

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