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MathGroup Archive 2002

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Re: Accuracy and Precision

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36872] Re: Accuracy and Precision
  • From: Bill Rowe <listuser at earthlink.net>
  • Date: Mon, 30 Sep 2002 03:03:20 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Neither SetAccuracy[expr,n] nor SetPrecisions[expr,n] modify expr. These functions modify the prinout not the internal representation. So, the first computation of f is done with approximate numbers and doesn't result in a correct answer due to approximate arithmetic.

By assigning a rational expression to each of the variables, you have made them exact numbers and Mathematica responds with an exact solution.

On 9/29/02 at 2:55 AM, pkosta2002 at yahoo.com (Peter Kosta) wrote:

>Could someone explain what is going on here, please?
>
>In[1]:= a = 77617.; b = 33096.;
>
>In[2]:= SetAccuracy[a, Infinity]; SetAccuracy[b, Infinity];
>SetPrecision[a, Infinity]; SetPrecision[b, Infinity];
>
>In[4]:= f := 333.75*b^6 + a^2*(11*a^2*b^2 - b^6 - 121*b^4 - 2) +
>5.5*b^8 + a/(2*b)
>
>In[5]:= SetAccuracy[f, Infinity]; SetPrecision[f, Infinity];
>
>In[6]:= f
>
>Out[6]= -1.1805916207174113*^21
>
>In[7]:= a = 77617; b = 33096;
>
>In[8]:= g := (33375/100)*b^6 + a^2*(11*a^2*b^2 - b^6 - 121*b^4 - 2) +
>(55/10)*b^8 + a/(2*b)
>
>In[9]:= g
>
>Out[9]= -(54767/66192)
>
>In[10]:= N[%]
>
>Out[10]= -0.8273960599468214
>
>Thanks,
>
>PK
>


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