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MathGroup Archive 1995

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Re: 2 questions about simplifications of expressions in MMA

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg438] Re: [mg423] 2 questions about simplifications of expressions in MMA
  • From: olness at phyvms.physics.smu.edu (Fredrick Olness (214) 768-2500 or -2495, Fax -4095)
  • Date: Mon, 30 Jan 1995 11:36:44 -0600

>  Fedor Pikus writes: (pikus at physics.ucsb.edu)
>  Dear Mathgroupers,
>  I've done some computations on MMA, and the results seem to be ok, but
>  writter in a horrible form.
>  First of all, how do I make MMA understand that  this
>  
>                                   2
>                e1 - Sqrt[(e1 - e2) ] - e2
>  Out[12]= {----------------------------------- }
>                                     2       2
>            Sqrt[(e1 - Sqrt[(e1 - e2) ] - e2) ]
>  
>  is really 1.
--------------------------------------------------------------------
PowerExpand will do much of what you need, but it assumes the opposite
branch of the Sqrt[] function than you want. 
--------------------------------------------------------------------
  In[]:= tmp= (e1-Sqrt[(e1-e2)^2] - e2 )/( Sqrt[(e1-Sqrt[(e1-e2)^2] - e2 )^2] )
  In[]:= tmp  //PowerExpand
  Out[]= 0
--------------------------------------------------------------------
The solution is to lead Mathematica through to the proper branch, and
it will do the rest. 
--------------------------------------------------------------------
  In[]:= tmp /.{Sqrt[(e1-e2)^2] -> -(e1-e2)} //PowerExpand
  Out[]= 1
--------------------------------------------------------------------
Fredrick I. Olness

SMU  Mail:  Department of Physics
            Fondren Science Bldg.
            Southern Methodist University
            Dallas, TX 75275

Internet:   Olness at phyvms.physics.smu.edu (129.119.200.74)
            Olness at mail.physics.smu.edu



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