It is easy to demonstrate that amplitude modulation satisfies the superposition principle, whereas angle

modulation does not. To be specific, let m₁(t) and m₂(t) be two message signals, and let u₁(t) and u₂(t)

be the corresponding modulated versions. Show that if m₁(t) + m₂(t) frequency modulates a carrier, the

modulated signal is not equal to u₁(t) + u₂(t).