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MathGroup Archive 1990

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Re:Does anyone have an apparent fix for this mathematica problem?

  • To: mathgroup at yoda.ncsa.uiuc.edu
  • Subject: Re:Does anyone have an apparent fix for this mathematica problem?
  • From: uunet!mathnx.byu.edu!smithw (William V. Smith)
  • Date: Thu, 7 Jun 90 22:16:39 MDT

Yes.  There is a fix.  It is available from your local friendly NeXT
Rep.  Here's what happens with the fix:

In[1]:= InputForm[(b*c*(a + d - (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))^3)/
                   (4*(a - d + (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))*
                       (a^2 + 4*b*c - 2*a*d + d^2)^(1/2)) -
               (b*c*(a +                 d + (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))^
3)/
                  (4*(a - d                 - (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))
*
                       (a^2 + 4*b        *c - 2*a*d + d^2)^(1/2))]

Out[1]//InputForm=
  (b*c*(a + d - (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))^3)/
    (4*(a^2 + 4*b*c - 2*a*d + d^2)^(1/2)*
      (a - d + (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))) -
   (b*c*(a + d + (a^2 + 4*b*c - 2*a*d + d^2)^(1/2))^3)/
    (4*(a^2 + 4*b*c - 2*a*d + d^2)^(1/2)*
      (a - d - (a^2 + 4*b*c - 2*a*d + d^2)^(1/2)))

In[2]:= rule={a->1,b->2,c->3,d->4}

Out[2]= {a -> 1, b -> 2, c -> 3, d -> 4}

In[3]:= N[res//.rule]

Out[3]= res

In[4]:= res=%1;

In[5]:= N[res//.rule]

Out[5]= 37.

In[6]:= N[Together[Expand[res]]//.rule]

Out[6]= 37.

In[7]:= N[Simplify[res]//.rule]

Out[7]= 37.


    The bad result was from a known bug in the Together function
which is called by Simplify so both gave the wrong answer.


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