Consider two real-valued signals si(t) and s2(t) whose pre-envelopes are denoted by S1+(t) and s2+(1), respectively. (a) Please show that ** Re[s4 (1)]Re[$+ (1)]dt ==Re [S (1)]s (1)dt ] 2+ (b) Following from (a) and assume that s(t) is a narrowband signal with complex envelop s (t) and carrier frequency fc, please show that 1 [s (1)dt = = [*]s(t)| dt 2