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Re: f'[0]=0.5 is True?

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  • Subject: [mg131402] Re: f'[0]=0.5 is True?
  • From: "Nasser M. Abbasi" <nma at 12000.org>
  • Date: Sat, 20 Jul 2013 05:57:13 -0400 (EDT)
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  • Reply-to: nma at 12000.org

On 7/18/2013 1:43 AM, mariusz sapinski 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[0] == 1;
> eqn2 = f'[0] == 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[0]==1,True}. >>
>
> so f'[0]=0.5 is True for Mathematica?
>
>  How can it be?
>

no problem in V 9.01, are you sure you are showing what you typed?

--------------------------
Clear[f];
Clear[x];
eqn = f''[x] + 2 f'[x] + 30 f[x] == 0;
eqn1 = f[0] == 1;
eqn2 = f'[0] == 0.5;
DSolve[{eqn, eqn1, eqn2}, f[x], x]

Out[18]= {{f[x] ->
    1. E^-x (1. Cos[Sqrt[29] x] + 0.278543 Sin[Sqrt[29] x])}}

---------------------------------
  
> If I remove eqn2 from DSolve then I get a solution with a parameter of course.
>
> Cheers,
>
>     Mariusz
>




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