Show that if ϕ = 0, but ωc –  = Δ, then for bandlimited inputs such that ωc > ωb the ou...

Show that if ϕ = 0, but ωc –  = Δ, then for bandlimited inputs such that ωc > ωb the output of the demodulator is

This result assumes that the cutoff frequency of the lowpass filter in (12.26) satisfies ωco > ωb + Δ.