solving a cubic equation

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vaggelis vaggelakis
评论： Christopher Creutzig 2014-8-29
I am using the command
x = solve('a*x^3 + b*x^2 + c*x + d')
to get the polynomial's roots. It returns a symbolic answer. Then i evaluate the a,b,c,d and i do copy-paste the first symbolic answer and then "enter" to get a numerical answer.
The numbers i get (1 almost real and 2 complex, as it is expected) are not roots of the equation!
I am looking for the poles of a mechanical system, so the roots must exist... What am i doing wrong?
  1 个评论
Christopher Creutzig
Christopher Creutzig 2014-8-29
Please provide the values for a, b, c, d with which you run into that problem.
Note that these solution formulas are well known to be numerically unstable. That may be the root of your problem. (No pun intended.)

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Star Strider
Star Strider 2014-8-20
Maybe I’m missing something, but why use the Symbolic Math Toolbox at all for this?
Use the roots function instead:
x = roots([a b c d])

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