function ex404
    clear;
    tspan=[0,100];
    t0=[50;1];
    [t,y]=ode45(@rv,tspan,t0);
    plot(t,y);
    grid('on');
    xlabel('Generation');ylabel('Number');
function dydt=rv(t,y)
    a=0.5;b=0.02;c=0.5;d=0.01;
    dydt=[a*y(1)-b*y(1)*y(2);-c*y(2)+d*y(1)*y(2)];