in an DSB or AM signal the frequency is kept constant ,only the amplitude(of the carrier) is varied in accordance with the modulating signal, than how can there be frequency (fc+fm) and (fc-fm)which is the frequency of the sidebanbs (if this is true ,it means that in the wave form of the AM signal the frequency will be varying) .According to the me the frequency should be only fc (If the frequency is changing than it is some type of FM wave.).so tell me the how the frequency of the side band have come.