Re: f'[0]=0.5 is True?
- To: mathgroup at smc.vnet.net
- Subject: [mg131404] Re: f'[0]=0.5 is True?
- From: Bob Hanlon <hanlonr357 at gmail.com>
- Date: Sat, 20 Jul 2013 05:57:53 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- Delivered-to: l-mathgroup@wolfram.com
- Delivered-to: mathgroup-outx@smc.vnet.net
- Delivered-to: mathgroup-newsendx@smc.vnet.net
- 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[0] == 1; eqn2 = f'[0] == 0.5; sol[x_] = f[x] /. DSolve[{eqn, eqn1, eqn2}, f[x], x][[1]] (1.*(1.*Cos[Sqrt[29]*x] + 0.2785430072655778* Sin[Sqrt[29]*x]))/E^x sol[0] 1. sol'[0] 0.5 Using exact numbers eqn2 = f'[0] == 1/2; sol[x_] = f[x] /. DSolve[{eqn, eqn1, eqn2}, f[x], x][[1]] ((1/58)*(58*Cos[Sqrt[29]*x] + 3*Sqrt[29]* Sin[Sqrt[29]*x]))/E^x sol[0] 1 sol'[0] 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[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? > > If I remove eqn2 from DSolve then I get a solution with a parameter of > course. > > Cheers, > > Mariusz > >
- References:
- f'[0]=0.5 is True?
- From: mariusz sapinski <mariusz.sapinski@gmail.com>
- f'[0]=0.5 is True?