MATLAB: How to plot the phase portrait of a second-order differential equation? I cannot use quiver

Question

?̈+ ?̇ − ? − ? 2 = 0

b. ?̈+ 4?̇ + 4? = 0

c. ?̈− 2?̇ + 2? = −8

d. ?̈− 2? − ?̇ − 2 = 0

I need to plot the phase portrait of these systems. What is wrong?

syms t

syms x(t)

dx=diff(x);

d2x=diff(x,2);

eq=d2x+dx-x-x.^2==0;

t=linspace(-1,1,100);

for n=1:5

s1=dsolve(eq,x(0)==n,dx(0)==n);

s2=diff(s1);

s1=subs(s1);

s2=subs(s2);

plot(s1,s2);

hold on

n=n+1;

end

Answer 1

Check the answer on similar question answered here