nnn=1:N;
figure(18)
 set(18,'Position',[10,50,500,200])
plot(nnn,s18);
axis([0,10*4*L,-4,4]);	
grid;


 s1818=s18(2*L:4*L:N);		  %取樣1
 nnn=1:M/4;
figure(7)
 set(7,'Position',[10,50,500,200])
stem(nnn,s7);
axis([0,10,-3,3]);	
grid;