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Re: Reduce::inex problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114377] Re: Reduce::inex problem
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Thu, 2 Dec 2010 05:42:00 -0500 (EST)

On 12/1/10 at 2:13 AM, wayo.cavazos at gmail.com (Eduardo  Cavazos)
wrote:

>This seems like a pretty simple set of equations, but it's stumping
>reduce:

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

>Of course, Solve can handle it. (Change the last expression to
>Solve[%,m].)

>Any thoughts on helping Reduce out here?

Yes. Don't use subscripted symbols. That is:

In[11]:= eqs = {Subscript[V, b] == 4/3*Pi*Subscript[r, b]^3,
    Subscript[V, a] == 4/3*Pi*Subscript[r, a]^3,
    V == Subscript[V, b] - Subscript[V, a], \[Rho] == m/V};

In[12]:= eqs /. {Subscript[r, b] -> 5.75,
   Subscript[r, a] -> 5.70, \[Rho] -> 8.92, Subscript[V, a] -> va,
   Subscript[V, b] -> vb}

Out[12]= {vb\[LongEqual]796.328,va\[LongEqual]775.735,V\[LongEqual]vb-va,8.92\[LongEqual]m/V}

In[13]:= Reduce[%]

Out[13]= vb == 796.328 && va == 775.735 && V == 20.5937 && m ==
8.92 V

You can get Mathematica to work with subscripted symbols
properly by using the functions in the Notation package. And
unless you use the Notation package you will run into
difficulties using subscripted symbols every now and then.



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