% Matlab第二章例2.9; 
%
a=[1,-1,0.9];b=1;
% Part a)
x=impseq(0,-20,120);n=[-20:120];
h=filter(b,a,x);
subplot(2,1,1);stem(n,h)
axis([-20,120,-1.1,1.1])
title('脈沖響應');text(125,-1.1,'n');ylabel('h(n)')
%
% Part b)
x=stepseq(0,-20,120);
s=filter(b,a,x);
subplot(2,1,2);stem(n,s)
axis([-20,120,-.5,2.5])
title('階躍響應');xlabel('n');ylabel('s(n)')
%
%print -deps2 ex021000.eps
%
% Part c)
sum(abs(h))
z=roots(a);
magz=abs(z)
subplot