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