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Re: Solve's Strange Output

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89093] Re: Solve's Strange Output
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sun, 25 May 2008 06:26:28 -0400 (EDT)
  • References: <200805250604.CAA15590@smc.vnet.net>

On 25 May 2008, at 15:04, Bruce Colletti wrote:

> Re 6.0.2 under WinXP.
>
> This code's output is strange:  what does 0.-7.9424 g mean?  Ditto  
> for all values returned by Solve.
>
> Thankx.
>
> Bruce
>
>
> {x[0],y[0]}={10.5,6.08};
> {x[1],y[1]}={3.23,14.4};
> {x[2],y[2]}={18,12.7};
> m=16.1;
>
> Solve[{a+c==0,b+d==m*g,d(x[2]-x[0])==c(y[2]-y[0]),a(y[1]- 
> y[0])==b(x[1]-x[0])},{a,b,c,d}]
>
> Out[11]= {{a->0.-7.9424 g,b->0.+9.08951 g,c->0.+7.9424 g,d- 
> >0.+7.01049 g}}
>


0. is just machine precision (hence approximate) 0, and

In particular, for an undefined symbols a an approximate 0 does not  
behave like genuine 0:

0. + a
  a + 0.
0.*a
0. a

This is reasonable since 0. + a could not be a, as

Precision[0. + a]
MachinePrecision

while

Precision[a]
Infinity

Andrzej Kozlowski


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