"Assume that Nyquist rate for signal X1( t) is fa.What is the Nyquist rate for the signal X2(t)=X1(t) * cos(2*pi*fb*t)?"
I have no idea how to do this question,the only thing I can deduce is that frequency of signal X1(t) is fa/2 but thats about it.Does it have anything to do with trigonometric identities?