```matlab
% 1 求离散系统的单位脉冲响应
%例如 : x(n)=y(n)-y(n-1)+0.9y(n-2)
x=1;
y=[1,-1,0.9];
impz(x,y,50);%求解并绘图
% 2 求解上述系统的幅频特性和相频特性
w=pi*freqspace(100);%f = freqspace(N) 返回一维频率向量 f 并假定围绕单位圆有 N 个等间距点
freqz(x,y,w);% 求解幅频特性和相频特性且制图
% 3 卷积运算
x2=[ 0.0181,0.0543,0.0543,0.0181];
h2=[1.000,-1.76,1.1829,-0.2781];
y2=conv(x2,h2);
n=-3:3;
stem(n,y2,'.k');