4.2-3 Determine and sketch the spectrum of the DSB-SC signal 2m(1) cos 4000nt for the following message signals: \text { (a) } m(t)=\operatorname{sinc}^{2}(100 \pi t-50 \pi) \text { : }

\text { (b) } m(t)=40 /\left(t^{2}-4 t+20\right) \text { . }