绘制包络线

x=0.:pi/8:4*pi;
y=exp(-x/3).*sin(3*x);
yb=exp(-x/3);
plot(x,yb,'k-',x,-yb,'k-',x,y,'-ro','linewidth',2,'markeredgecolor','g','markerfacecolor','y','markersize',6)
grid on