how i can resolve the Error in sym/subsref (line 870) R_tilde = builtin('s​ubsref',L_​tilde,Idx)​;

clc, clear all, close all

syms x %define the vairable of(x)

f=-1.8*x^4+6.03*x^3+57.03*x^2-171.41*x+30.30;

figure(1)

fplot(f);

xL = xlim;

yL = ylim;

line(xL, [0 0],'color','k','linewidth',2) %x-axis

title('plot of Polynomial function');

xlabel('X-axis');

ylabel('Y-axis');

hold on

%Find roots by using built in symbolic command

disp('Matlab built in Symbolic command (roots function)')

d=[-1.8,6.03,57.03,-171.41,30.30] %vector of coefficients

sol1=roots(d) %solve by roots function

plot(sol1,'ko') %draw solution

%Define regula falsi method as function

[xr,ir]=regulafalsi(f,3,7,1e-5,100);

function [c,i] =regulafalsi(f,a,b,delta,N)

% Regula falsi method

c=(a*f(b)-b*f(a))/(f(b)-f(a)); % 1st iteration

i=1; % number of iteration

% Stopping Criteria:

% error is smaller then the given tolerance, or the maximum iteration number is reached

while abs (f(c)) > delta && i <= N

if f(c)*f(a) < 0

b = c;

else

a = c;

end;

i = i + 1;

c = (a*f(b) - b*f(a))/(f(b) - f(a));

end;

end

Answer 1

[Walter Roberson]

f=-1.8*x^4+6.03*x^3+57.03*x^2-171.41*x+30.30;

That is a scalar expression, not a function. f(a) would be a request to index it.

Instead of assigning to f, assign to f(x) to make it into a function.