Re: solve for a squared variable

*To*: mathgroup at smc.vnet.net*Subject*: [mg60469] Re: solve for a squared variable*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Sat, 17 Sep 2005 02:31:50 -0400 (EDT)*Organization*: The Open University, Milton Keynes, U.K.*References*: <200509150916.FAA15860@smc.vnet.net> <dgduk1$19t$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Ruth Lazkoz wrote: > Hi everyone, > > Why does > > Solve[x^2 + y == 1, x^2] > > give and error message? How can I solve for x^2? > > Thanks, > > Ruth > Hi Ruth, Since the error message is _General::ivar : x^2 is not a valid variable. More..._ so we must do a change of variable as in the following example: In[1]:= expr = x^2 + y == 1 Out[1]= x^2 + y == 1 In[2]:= Solve[expr /. x^2 -> X, X] Out[2]= {{X -> 1 - y}} In[3]:= x -> Sqrt[X /. %[[1]]] Out[3]= x -> Sqrt[1 - y] In[4]:= expr /. % Out[4]= True Regards, /J.M.

**References**:**Re: Why this function does not return a single value***From:*Bill Rowe <readnewsciv@earthlink.net>