% file: task21.m
% Filter with the frequency response:
%               0.5 - 0.5 e^-jw
%  H(e^jw) = --------------------
%                1  + 0.9 e^-jw
B = [0.5  -0.5];
A = [1  0.9];
figure(3); clf;
freqz(B, A);      % ampl.response + phase response
figure(4); clf;
impz(B, A);       % impulse response h[n]