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Spurious solution out of the blue

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131397] Spurious solution out of the blue
  • From: carlos.felippa%colorado.edu at gtempaccount.com
  • Date: Thu, 18 Jul 2013 03:01:47 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • Delivered-to: l-mathgroup@wolfram.com
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Running under Mathematica 5.2:

ClearAll[y,x2,x3]; 
y=(12*x2*(150 - x2 + 4*x3 + 4*Sqrt[30]*Sqrt[30 - x2 + 4*x3]))/
  (7*x2 - 8*(45 + 2*x3 + 2*Sqrt[30]*Sqrt[30 - x2 + 4*x3]))^2; y=Simplify[y];
sol=Simplify[Solve[y==0,x2]];  Print[sol]; Print[Simplify[y/.sol]];

There are two solutions but the nonzero one is incorrect. Where does the 
spurious solution come from?  I get the same result by solving
Numerator[y]==0.



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