clear all;
[T,Y] = meshgrid(0:0.25:5,800:10:1000);
S = 0.5*Y - 450;
L = sqrt(1 + S.^2);
quiver(T,Y,ones(size(L)),S,0.5,'.')
hold on
contour(T,Y,exp(-0.5*T).*(Y-900),[-40 -20 -10 -5 0 5 10 20 40])
axis([0,5.5,790,1010])
xlabel('t'),ylabel('y')
title('Directional field and integral curves for dy/dt = 0.5y-450')
%print -djpeg DirectionFieldSection1.1.jpg