roots

how to find a roots of a cubic equation in matlab?

Answer 1

[Andrei Bobrov]

please read about function roots

or use function solve from Symbolic Math Toolbox

eg

use roots

%polynomial x^3-6*x^2-50*x-4

p = [1 -6 -50 -4]
out = roots(p)

use solve and Symbolic Math Toolbox

syms x
out2 = solve(x^3-6*x^2-50*x-4)
vpa(out2,6)

on Nasir comment

out = zeros(3,...);
for j1 = 1:....
    B = ...
    A = ...
    out(:,j1) = roots([1, B-1, A-3*B^2, A*B-B^2-B^3]);
end

[Nasir Qazi]

if I have equation like this

Z^3 +(B-1)Z^2 +(A-3B^2)Z -(AB-B^2-B^3)=0

where the values of A & B are varies in iteration .

in that case how can I calculate the Roots ?

[Walter Roberson]

Careful, there would be three roots for each cubic, so you would not be able to store the output of roots() in to a single array location out(j1)

[Andrei Bobrov]

Thank you Walter! Corrected.

[Nasir Qazi]

I have a condition of trail and error which will repeat the process of roots find after a condition of error checking , but after 10 iteration it give me a error , root input cannot be inf or nan , where exactly do I have a fault in the coding ... ?

[Walter Roberson]

I think we would need to see the code.