clc,clear
t=linspace(0,pi,20000);
y1=sin(t)'*[1,-1];  % 包絡線
y=sin(t).*sin(9*t);

% 求過零點
t0=t(find(abs(y)<1e-3));  
% t0=pi*(0:9)/9;  
y0=sin(t0).*sin(9*t0);
plot(t,y1,'r:',t,y,'b',t0,y0,'bo','markersize',10)
axis([0,pi,-1,1])