x=(0:pi/100:2*pi)';
y1=2*exp(-0.5*x)*[1,-1];
y2=2*exp(-0.5*x).*sin(2*pi*x);
plot(x,y1,'b:');
axis([0,2*pi,-2,2]);
hold on;
plot(x,y2,'k');
legend('包络线','包络线','曲线y');
hold off;
grid on;