A singnal x(t) = 100cos (24pi X 10³)t is ideally sampled with a sampling period of 50usec and then passed through an ideal lowpass filter with cutoff frequency of 15 KHz. which of the following frequencies is/are present at the filter output?

a) 12 KHz only

b) 8 KHz only

c) 12 KHz and 9 KHz

d) 12 KHz and 8 KHz

fs = 1 /Ts = 1/ 50 X 10 ^{-6 =}   20 khz

fc = 24  X 10 ³    /  2

The frequencies passed through LPF are fc , fs - fc i.e 12 khz and 8 khz