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MathGroup Archive 2012

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Re: "Solve" on polynomial equality results in expressions containing "Root"

  • To: mathgroup at smc.vnet.net
  • Subject: [mg128523] Re: "Solve" on polynomial equality results in expressions containing "Root"
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Sun, 28 Oct 2012 01:55:40 -0400 (EDT)
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On 10/26/12 at 11:48 PM, coder0xff at gmail.com wrote:

>If I do Solve[y==3x^2-2x^3,x] I get expressions containing imaginary
>parts. If I set the domain to the Reals
>(Solve[y==3x^2-2x^3,x,Reals]) then I got stuff like Root[y - 3 #1^2
>+ 2 #1^3 &, 2]. I know there's a solution using only reals (since
>the range and domain are both real, and complex math is performed as
>operations on reals after all) How can I get it to give me the
>actual solution using only reals?

Probably the simplest way to accomplish your goal is to let
Solve return all solutions and delete the complex solutions, i.e.,

Cases[Solve[y == 3 x^2 - 2 x^3, x], _?(FreeQ[#, Complex] &)]




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