0 个投票

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
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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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 采纳的回答

Star Strider
Star Strider 2014-8-20

6 个投票

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])

更多回答（1 个）

Walter Roberson
Walter Roberson 2025-7-7
Ran in:

0 个投票

Ran in:
syms a b c d x
sol = solve(a*x^3 + b*x^2 + c*x + d, x, 'maxdegree', 3)
sol = 
numsol = subs(sol, [a,b,c,d], [1.5, -2, 3/8, -5]) %substitute in actual values for a b c d
numsol = 
●
snumsol = simplify(numsol, 'steps', 50)
snumsol = 
double(snumsol)
ans =
2.0237 + 0.0000i -0.3452 - 1.2361i -0.3452 + 1.2361i

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