MATLAB: How can i solve this polynomial equation

determinants

clear all clc; syms x A=[x 2 1;1 0 -x;5 1 x]; det(A) r=solve('det(A)==0','x')

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syms x
A=[x 2 1;1 0 -x;5 1 x]
r=double(solve(det(A),x))

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MATLAB: What is the mistake

syms x
A=[x 2 1;1 0 -x;5 1 x];
x=-15:.1:15;
plot(x,subs(det(A),x))
grid on

MATLAB: How to fit custom polynomial equation

You could download polyfitn from the file exchange.

But if you already know how to use the curve fitting toolbox, then just specify a custom equation of the desired form. Something like this:

modelform = fittype('p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p30*x^3 + p21*x^2*y',...
    'coeff',{'p00','p10','p01','p20','p11','p02','p30','p21'},...
    'indep',{'x','y'},'depend','z');

It will be treated as a nonlinear model internally as a function of the parameters, but since all of the coefficients actually do enter linearly, they will be easy to fit. fit should have no problems.

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Related Solutions

MATLAB: What is the mistake

MATLAB: How to fit custom polynomial equation

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