繪製包絡線

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 　　

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