：A:b = [1 1];a = [1 -0.9 0.81];x = ones(1,100);y = filter(b,a,x);B:b = [1 1];a = [1 0.9 -0.81];x = ones(1,100);y = filter(b,a,x);答案: b = [1 1];a = [1 -0.9 0.81];x = ones(1,100);y = filter(b,a,x);17、