Help with selecting Nyquist rate

[mngeow]

Ok so I have this question about Nyquist rate:

"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?

Thanks alot!

 

[alec_t]

Hint: ask yourself 'what is the frequency of signal X2(t) ?'

 

[mngeow]

I have no idea. I know the frequency of the cosine wave that X1(t) is being multiplied by is fb,so the frequency of X2(t) should be fafb?