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MathGroup Archive 1999

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questions about delayed expression.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg19862] questions about delayed expression.
  • From: d8442803 at student.nsysu.edu.tw (Wen-Feng Hsiao)
  • Date: Sun, 19 Sep 1999 01:20:45 -0400
  • Organization: MIS
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

   The following is the process that I run in my notebook.

In[1]:=
a[x_] := x + 4
b[x_] := -3 x + 8

In[2]:=
sola = x /. Solve[{a[x] == c}, x];
solb = x /. Solve[{b[x] == c}, x];

In[3]:=
Inter[c_] := Interval[{sola[[1]], solb[[1]]}]        

In[4]:=
Inter[.3]
Inter[.4]

Out[4]=
\!\(Interval[{\(-4\) + c, \(8 - c\)\/3}]\)

Out[5]=
\!\(Interval[{\(-4\) + c, \(8 - c\)\/3}]\)

My questions are: 
1. Why Inter[.3] and Inter[.4] cannot be evaluated? Their results should 
not be the same. This is not my intention.

2. I don't know if there is any better way to extract the 'root(s)' from 
the output of 'Solve' command. The output form is {{x->root1}, {x-
>root2}, ...{}}. If I use 'ReplaceAll'(/.) command, it will remain a list 
of solutions of x. It seems I can only use element operation to extract 
the root(s) from the solution list? In my case, I use sola[[1]] and 
solb[[1]].
 
Please give me suggestions. Thanks for your help.


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