Re: Factor and/or Rules replacements
- To: mathgroup at smc.vnet.net
- Subject: [mg104499] Re: [mg104443] Factor and/or Rules replacements
- From: Bob Hanlon <hanlonr at cox.net>
- Date: Sun, 1 Nov 2009 17:57:22 -0500 (EST)
- Reply-to: hanlonr at cox.net
f = Times @@ (x - (x /. Solve[x^2 + b x + c == 0, x]))
((1/2)*(b - Sqrt[b^2 - 4*c]) + x)*
((1/2)*(Sqrt[b^2 - 4*c] + b) + x)
f // Expand
b*x + c + x^2
Bob Hanlon
---- yves <yves.dauphin at solvay.com> wrote:
=============
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