A low pass filter is described by the difference equation y(n) = 0.9y(n 1) + 0.1x(n)

Question:

A low pass filter is described by the difference equation y(n) = 0.9y(n – 1) + 0.1x(n)
(a) By performing a frequency translation of π/2, transforms the filter into a band pass filter.
(b) What is the impulse response of the band pass filter?
(c) What is the major problem with the frequency translation method for transforming a prototype low pass into a band pass filter?