4.2.2 A given voltage signal is f(t) 4 cos 20πt + 2 cos 30πt across 1. a) Determine the power spectral density of f(t). b) Sketch S,(w). c) Calculate the mean (average) power, both in the time domain and in the fre- quency domain, that is dissipated by f(t) across the one-ohm resistor.