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Re: Solving equations and inequalities with Reduce - how?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg87692] Re: Solving equations and inequalities with Reduce - how?
  • From: dh <dh at metrohm.ch>
  • Date: Tue, 15 Apr 2008 06:50:32 -0400 (EDT)
  • References: <fu1tuf$ogc$1@smc.vnet.net>


Hi Marc,

your syntax is correct. But you met a common pit fall. On of your 

variables already had a value (of 1) and therefore got replaced by this 

value before Reduce got it. You may clear a value by e.g. a=. or Clear[a].

hope this helps, Daniel



Marc Heusser wrote:

> I tried to solve equations with Reduce and somehow did not quite 

> formulate it right, so Reduce complains with 

> "Reduce::ivar: 1 is not a valid variable".

> 

> This is what I tried:

> 

> Wanted: A six digit number satisfying the following conditions:

> The first digit is not zero.

> If you take the first two digits and move them to the end of the number, 

> the resulting number must be twice the original number.

> 

> In[23]:=Reduce[200000a +20000 b +2000 c+200 d+20 e  +       2 f 

> \[Equal]100000c +10000 d +1000 e+100 f+10 a  + b ,  {a,b,c,d,e,f}, 

> Modulus\[Rule]9]

> 

> I did solve the problem through exhaustive search:

> 

> In[14]:=Timing[Select[Range[10^6],    FractionalPart[#/10000] 1000000 + 

> IntegerPart[#/10000]\[Equal] 2  #&]]

> Out[14]={34.2848 Second,{142857,285714,428571}}

> 

> but would like to understand how to use Reduce (or another function) to 

> solve such a set of equations.

> 

> TIA

> 

> Marc

> 




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