Why does an error saying "Unrecognised function or variable" occrurs while solving a two variable polynomial equation numerically using vpasolve

I am trying to solve a bi variable polynomial equation x^4+3*x^2+x*t==0, where t is an independent variable and x is the dependent variable and I want to plot the results as well. So I use the help of vpasolve to get the results over a range of values for the independent variable 't'. I used the following code

clc;clear;close all;
sym x;
sym t;
t= 0.1:0.01:1;
for i= length(t)
    S(i)= vpasolve(x^4+3*x^2+x*t==0,x);
end

Unrecognized function or variable 'x'.

plot(x,t)

If I run the above code I receive the following error

Unrecognized function or variable 'x'.

Error in sample_polynomial_equation_trial (line 6)
    S(i)= vpasolve(x^4+3*x^2+x*t==0,x);
    

What correction should be done in order to get the code running? I am using MATLAB version R2020a

Answer 1

[Star Strider]

Needs parentheses, single quotes, and some other tweaks —

% clc;clear;close all;
x = sym('x');
% sym(t);
t= 0.1:0.01:1;
for i= length(t)
    S(:,i)= vpasolve(x^4+3*x^2+x*t(i)==0,x);
end

figure
subplot(2,1,1)
plot(t,real(double(S)))
grid
xlim([0.99 1.00])
subplot(2,1,2)
plot(t, imag(double(S)))
grid
xlim([0.99 1.00])

.