       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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