clc
T=0.001;
t=[0:T:8];u=sin(t*pi/2);
y=zeros(size(u));
for k=1:fix(8/T)+1;
    if k==1
        y(k)=u(k);
    elseif k==2
        y(k)= u(k)+0.8*y(k-1);
    else
        y(k)=u(k)+0.8*y(k-1)-2.5*y(k-2);
    end
end
time=toc
figure(1)
subplot(2,1,1)
plot(t,u),title('Input u')
subplot(2,1,2)
plot(t,y),title('Output y')