t1=-2;t2=2;x1=0;x2=5;
[t,x]=meshgrid(t1:0.2:t2,x1:0.5:x2);
f=-x.^1/2+2.*t.^2;
dt=cos(atan(f));
dx=sin(atan(f));
quiver(t,x,dt,dx,.35);

t1=0;t2=15;x1=0;x2=7;
[t,x]=meshgrid(t1:1:t2,x1:.5:x2);
f=.5*x.*(1-(t-.5).*x/20);
dt=cos(atan(f));
dx=sin(atan(f));
quiver(t,x,dt,dx,.5);

warning off MATLAB:divideByZero
clear all
b=.5;d=.55;
t1=0;t2=1.2;x1=0;x2=1.2;
[t,x]=meshgrid(t1:.1:t2,x1:.1:x2);
f=t.*(1-t-x./(b+t));g=(x.*(t./(b+t)-d));
dt=f./sqrt(f.^2+g.^2);
dx=g./sqrt(f.^2+g.^2);
quiver(t,x,dt,dx,.35);


warning off MATLAB:divideByZero
clear all
b=.5;d=.55;
t1=0;t2=1.2;x1=0;x2=1.2;
[t,x]=meshgrid(t1:.1:t2,x1:.1:x2);
dt=t.*(1-t-x./(b+t));dx=(x.*(t./(b+t)-d));
quiver(t,x,dt,dx,1.5);
