       Re: f'=0.5 is True?

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• Subject: [mg131404] Re: f'=0.5 is True?
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Sat, 20 Jul 2013 05:57:53 -0400 (EDT)
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• References: <20130718070007.6E53169E5@smc.vnet.net>

```Try starting with a fresh kernel.

Clear[f, x, sol];

eqn = f''[x] + 2 f'[x] + 30 f[x] == 0;
eqn1 = f == 1;
eqn2 = f' == 0.5;

sol[x_] = f[x] /. DSolve[{eqn, eqn1, eqn2}, f[x], x][]

(1.*(1.*Cos[Sqrt*x] + 0.2785430072655778*
Sin[Sqrt*x]))/E^x

sol

1.

sol'

0.5

Using exact numbers

eqn2 = f' == 1/2;

sol[x_] = f[x] /. DSolve[{eqn, eqn1, eqn2}, f[x], x][]

((1/58)*(58*Cos[Sqrt*x] + 3*Sqrt*
Sin[Sqrt*x]))/E^x

sol

1

sol'

1/2

Bob Hanlon

On Thu, Jul 18, 2013 at 3:00 AM, mariusz sapinski <
mariusz.sapinski at gmail.com> wrote:

> Dear All,
>
> I'm trying a simple exercise:
>
> eqn = f''[x] + 2 f'[x] + 30 f[x] == 0;
> Clear[f];
> Clear[x];
> eqn1 = f == 1;
> eqn2 = f' == 0.5;
> DSolve[{eqn, eqn1, eqn2}, f[x], x]
>
>
> and I get:
> DSolve::deqn: Equation or list of equations expected instead of True in
> the first argument {30 f[x]+2
> (f^\[Prime])[x]+(f^\[Prime]\[Prime])[x]==0,f==1,True}. >>
>
> so f'=0.5 is True for Mathematica?
>
> How can it be?
>
> If I remove eqn2 from DSolve then I get a solution with a parameter of
> course.
>
> Cheers,
>
>    Mariusz
>
>

```

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