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Re: why is this happening?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99225] Re: why is this happening?
  • From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
  • Date: Thu, 30 Apr 2009 06:23:11 -0400 (EDT)
  • References: <gt9aq0$q7o$1@smc.vnet.net>

Sean,

After backsubstitution the answers Mathematica provides seem to be A-
OK, so what's the problem?

In[517]:=
k1 ld + k1 k2 + ld^2 == 0 /.
  Solve[k1 ld + k1 k2 + ld^2 == 0, ld] // Simplify

Out[517]= {True, True}

In[516]:=
k2/k1 + ld^2 + ld == 0 /. Solve[k2/k1 + ld^2 + ld == 0, ld] //
Simplify

Out[516]= {True, True}


Cheers -- Sjoerd

On Apr 29, 12:40 pm, sean_inc... at yahoo.com wrote:
> Consider the following.
>
> Solve[k1 ld + k1 k2 + ld^2 == 0, ld]
>
> vs
>
> Solve[k2/k1 + ld^2 + ld == 0, ld]
>
> I expected to get the output I got from the first code.
>
> >From the second code, I was expecting something along the lines of
>
> ld ->-1 + (SquareRoot[1 - 4 (k2/k21)])/2
> ld -> -1 - (SquareRoot[1 - 4 (k2/k21)])/2
>
> since that is the answer.
>
> Why is Mathematica turning the second scaled equation into the
> unscaled one in the first before solving it?
>
> Thanks for any insight.
>
> Sean



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