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MathGroup Archive 2006

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Re: Two questions (1) Sollve and (2) Precision

  • To: mathgroup at smc.vnet.net
  • Subject: [mg67103] Re: Two questions (1) Sollve and (2) Precision
  • From: albert <awnl at arcor.de>
  • Date: Fri, 9 Jun 2006 01:08:48 -0400 (EDT)
  • References: <e666j8$nbj$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Bharat,
 
> (1) I was trying to solve the follwing two linear equations using 'Solve'.
> *In:
> Solve[{64919121*x-159018721*y=8A1,41869520.5*x-102558961*y=8A0},{x,y}]*
> 
> *Out: {}*

others have given pointers, but since I have been typing variants that solve
your problem in to a notebook I thought I could as well send them:

this one rationalizes all numbers, so works with exact numbers and gives the
correct result:

Solve[Rationalize@{64919121*x - 159018721*y == 1, 
      4.18695205*10^7*x - 102558961*y == 0}, {x, y}]

this is the input required to _start_ with 50 digits of precision, and again
the result is correct to the desired precision:

Solve[{64919121*x - 159018721*y == 1, 
    4.18695205`50*10^7*x - 102558961*y == 0}, {x, y}]

I'm not an expert in this, but I think the problem here is that it takes
some care to use Solve and some other functions which are designed to work
on exact input with inprecise numbers. And of course the correct syntax
helps, too :-). If you search the archive you might find more qualified
posts about the topic.

hth

albert


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