I came across a question asking for modulated and demodulated signals whereby the given signal is x(t) = cos(2pi10t),

and the carrier signal is c(t) = cos(2pi100t).

I know that to get the modulated signal I need to multiply m(t) = x(t) * c(t) to get maybe frequencies of 90Hz and 110Hz.

To get demodulated signal I need to multiply d(t) = m(t) * c(t).

I don't understand the way of multiplication this part and maybe anyone can help?

Also, how can we recover the original signal from that d(t) and how do I go about setting the parameters?

Sorry but this kind of questions may seems noob to most you guys, but I'm just started learning these signal processing thingy and got quite a headache in the calculation part. Hope you guys could help =)