Trouble with For Loop in Differential Equations

clear all

%Problem 1a

syms x(t) Fmag;

Dx=diff(x);

k=36;

Fmag=4*cos(4*t);

disp(['dx^2/d^2t +' num2str(k) 'x=' char(Fmag)])

x(t)=simplify(dsolve(diff(x,t,2)+k*x==Fmag,x(0)==0,Dx(0)==4));

dx(t)=diff(x(t));

%Problem 1b

fplot(x(t))

title('Graph of x(t)')

for zeros=dx(t)==0

x(zeros)

end

%Problem 2a

syms w Fmag2;

Fmag2=4*cos(w*t);

disp(['dx^2/d^2t +' num2str(k) 'x=' char(Fmag2)])

x(t)=dsolve(diff(x,t,2)+36*x==Fmag2,x(0)==0,Dx(0)==4);

%Problem 3

%syms x3(t);

c=2;

disp(['dx^2/d^2t +' num2str(c) 'dx/dt +' num2str(k) 'x=' char(Fmag)])

fplot(x(t))

title('Graph of x(t) for Problem 3')