Path: news.mathworks.com!not-for-mail
From: "Steven_Lord" <slord@mathworks.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Haw to find symbolick roots
Date: Fri, 23 Sep 2011 16:41:36 -0400
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"kamuran turksoy" <kamuranturksoy@gmail.com> wrote in message 
news:j5inu6$nni$1@newscl01ah.mathworks.com...
> "Steven_Lord" <slord@mathworks.com> wrote in message 
> <j5i2c3$pe6$1@newscl01ah.mathworks.com>...
>>
>>
>> "kamuran turksoy" <kamuranturksoy@gmail.com> wrote in message 
>> news:j5g5jo$mr8$1@newscl01ah.mathworks.com...
>>
>> *snip*
>>
>> > Seems i have roughly same problem, in my case, i use
>> >
>> > X=solve('a*x^4+b*x^3+c*x^2+d*x+1=0', 'x'), but it gives result as:
>>
>> Do not call SOLVE with strings as input; SOLVE can't substitute values 
>> that you may have defined for the variables into the 
>> expression-to-be-solved if you do, and so in your example treated a, b, 
>> c, and d as symbolic variables rather than the specific values you'd 
>> already defined. Call SOLVE with symbolic expressions as input instead.
>>
>> syms x
>> X=solve(a*x^4+b*x^3+c*x^2+d*x+1, x) % the = 0 is implied
>>
>> -- 
>> Steve Lord
>> slord@mathworks.com
>> To contact Technical Support use the Contact Us link on 
>> http://www.mathworks.com
>
> But i want to solve it as symbolic, i mean  a b c d x are all symbols and 
> then my aim is to define some constraints on a b c d such that roots of 
> polynomial lie outside unit circle (abs (roots)> 1)

While there is a formula for the roots of a quartic equation in radicals, 
it's VERY messy.

http://en.wikipedia.org/wiki/Quartic_function#The_general_case.2C_along_Ferrari.27s_lines

I'm not all that surprised SOLVE doesn't try to expand it.

-- 
Steve Lord
slord@mathworks.com
To contact Technical Support use the Contact Us link on 
http://www.mathworks.com