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Re: Reduce::inex problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114361] Re: Reduce::inex problem
  • From: Eduardo Cavazos <wayo.cavazos at gmail.com>
  • Date: Thu, 2 Dec 2010 05:39:01 -0500 (EST)

On Wed, 2010-12-01 at 08:47 +0100, Andrzej Kozlowski wrote:

> Clearly Reduce does not like subscripts. 
>  {Vb == (4*Pi*rb^3)/3, Va == (4*Pi*ra^3)/3, 
>   V == Vb - Va, \[Rho] == m/V}; 
> 
> % /. {rb -> 5.75, ra -> 5.7, \[Rho] -> 8.92}; 
> 
>  Reduce[%]
> 
> Vb == 796.3282878380627 && Va == 775.7346243950061 && 
>    V == 20.593663443056585 && m == 8.92*V

Ah yes... I've seen this before. Thanks for reminding me. :-)

Using subscripts can seemingly trip up Reduce sometimes. In this case,
the problem comes from mixing subscripted and non-subscripted V.

Here's a version with the subscripts which works; I just changed the 'V'
to an 'A':

{
  Subscript[V, b] == 4/3*Pi*Subscript[r, b]^3,
  Subscript[V, a] == 4/3*Pi*Subscript[r, a]^3,
  A == Subscript[V, b] - Subscript[V, a],
  \[Rho] == m/A
  };
% /. {Subscript[r, b] -> 5.75, 
   Subscript[r, a] -> 5.70, \[Rho] -> 8.92};
Reduce[%, {m, A}]

And here's another where I changed 'V' to Subscript[V,total]. I.e. all
the instances of 'V' have subscripts:

{
  Subscript[V, b] == 4/3*Pi*Subscript[r, b]^3,
  Subscript[V, a] == 4/3*Pi*Subscript[r, a]^3,
  Subscript[V, total] == Subscript[V, b] - Subscript[V, a],
  \[Rho] == m/Subscript[V, total]
  };
% /. {Subscript[r, b] -> 5.75, 
   Subscript[r, a] -> 5.70, \[Rho] -> 8.92};
Reduce[%, {m, Subscript[V, total]}]

Thanks again Andrzej.

Ed



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