Q5. State weather each of the following is valid. If not, give at least one reason. a) Sxx() is a valid power spectral density. = b) Sxx() exp[-(-1)] is a valid power spectral density. = e) Rxx(7) 5 sin(37) is a valid autocorrelation function. d) The following is a valid autocorrelation function t 1+2 for 1 <