solving a cubic equation

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?

[Christopher Creutzig]

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.)

Answer 1

[Star Strider]

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])

[Sergio E. Obando]

This doc page also offers a discussion on "Roots of Polynomials"

Answer 2

[Walter Roberson]

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