clear
a=1;b=1;c=1;d=1;
h=0.01
N=2000

function z=f1(x,y)
  z=x*(a-b*y)
endfunction

function z=f2(x,y)
 z=y*(c*x-d)
endfunction

S(1)=5;R(1)=2;
for k=1:N
  s1=f1(S(k),R(k));
  s2=f2(S(k),R(k));
  s11=f1(S(k)+h/2*s1,R(k)+h/2*s2);
  s22=f2(S(k)+h/2*s1,R(k)+h/2*s2);
  S(k+1)=//.....à compléter
  R(k+1)=//.....à compléter
end;
  
xset("window",1)
plot2d(S,R)
