3.8 求下列信号的傅里叶变换:-|||-(a) (t)=(e)^-atcos omega t(t),(agt 0);-|||-(b) (t)=(e)^2+t(-t+1);-|||-(c)f(t)=2a^k ∅(t-kT),(|a|,t 为其他值;-|||-(f) (t)=cos omega t(t)