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MathGroup Archive 2009

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Factor and/or Rules replacements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg104443] Factor and/or Rules replacements
  • From: yves <yves.dauphin at solvay.com>
  • Date: Sun, 1 Nov 2009 03:56:16 -0500 (EST)

Hello everybody,

Here is the model problem:
I tried to obtain the factorisation of x^2+b x +c and expected to get something like (x-1/2(-b+Sqrt[b^2-4 c))((x-1/2(-b-Sqrt[b^2-4 c)).

So I tried 
in:Factor[x^2+b x + c] and get
out: c + b x + x^2
So my first question what should I have done?

I tried another way
in:sol=Solve[x^2+b x + c == 0,{x}]; and get the couple of replacement rules.
Next I defined
in: f=(x-x1)(x-x2)
and finally I replaced x1 and x2 in succession by
in:f /. {(sol[[1]] /. x -> x1), (sol[[2]] /. x -> x2)}
which gave the expected factorisation but enclosed in {}.
My second question is :
Is there a simpler way or more compact to replace x1 and x2?

With anticipated thanks.
Yves


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