How can i run correctly my Backward Difference Formula code ?

%% Backward Difference Formula Method %%
clc; clear all;
h=0.01;
t=0:h:1;
n=numel(t);
mu = 20;
f_m = @(t,y) mu*(y-cos(t))-sin(t);
exact = @(t) exp(mu*t)+cos(t);
%initials%
y_m(1)=exact(0);
y_m(2)=exact(h);
%Adam-Bashforth method%
x = ( 4*y_m(n-1)-y_m(n-2) )/3 + 2*h/3* ( mu*( x - cos(t(n)) ) - sin(t(n)) )
S=solve(x)
for i=3:n
 
   y_m(i)=(4.*y_m(i-1)-y_m(i-2))/3+(2/3).*h*(3*f_m(t(i)))
end

plot(t, exact(t)); 
hold
plot(t,y);
%plot(t,y,'-o');
legend('Exact Solution','BDF Solution')
xlabel('t')
ylabel('y')
title('When h = 0.01 and µ=20')

Answer 1

[VBBV]

%% Backward Difference Formula Method %%
clc; clear all;
h=0.01;
t=0:h:1;
n=numel(t);
mu = [20 -20]

mu = 1×2

20 -20

hold all
for k = 1:length(mu)
    f_m = @(t,y) mu(k)*(y-cos(t))-sin(t);
    exact = @(t) exp(mu(k)*t)+cos(t);
    %initials%
    y_m(1)=exact(0);
    y_m(2)=exact(h);
    %Adam-Bashforth method%
    syms y
    y = ( 4*y_m(1)-y_m(2) )/3 + 2*h/3* ( mu(k)*( y - cos(t(n)) ) - sin(t(n)) );
    S=solve(y,[0]);
    Y_m = [y_m zeros(length(t)-2,1).'];
    for i=3:n 
       Y_m(i)=(4.*y_m(1)-y_m(2))/3+(2/3).*h*(3*f_m(t(i),Y_m(i-1)));
    end
    subplot(211)
    plot(t, exact(t)); 
    subplot(212)
    plot(t,Y_m);
    
    hold on
        
end  
xlabel('t')
ylabel('y')
legend('\mu = 20','\mu = -20','location','best')