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MathGroup Archive 2002

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Re: 1 equals 3 (among others)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32200] Re: 1 equals 3 (among others)
  • From: Stegemann at physikDONOT.SPAMtu-berlin.de (Grischa Stegemann)
  • Date: Sat, 5 Jan 2002 00:10:43 -0500 (EST)
  • References: <a13v7l$dgb$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Fri, 4 Jan 2002 10:13:09 +0000 (UTC), Grischa Stegemann wrote:
> 
> In[1]:=f[x_] = (4 - x + Sqrt[-4*(3 - x) + (x - 4)^2])/2;
> In[2]:=Simplify[-4*(3 - x) + (x - 4)^2]
> Out[2]=(-2 + x)^2

Oh stupid fool I am! All the time I have dealt with complex numbers 
but only in this special case everything becomes real. So I have 
totally ignored that Sqrt[x^2]=Abs[x]. This of course explains the 
numbers I mentioned.

Nevertheless I found out that this wasn't actually my problem. The 
actual problem arised from the plot
Plot[f[x],{x,0,5}]
because I have neglected the numbers at the y-axis and thought that 
Mathematica is giving me lots of rubbish.
In this plot you can see numerical noise which can be hidden by 
e.g. giving PlotRange->{0,3}) but which doesn't appear if you plot
g[x_]=Sqrt[-4*(3 - x) + (x - 4)^2];
Plot[g[x],{x,0,5}].

OK, this is not really essential... ;-)


Thanks to all who pointed me to my stupidity.


Bye, Grischa
-- 
-------------------------------------------------------------------------
   Grischa Stegemann                     Technische Universität Berlin --



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