2-13 求下列各函数f1 (t)与f2(t )的卷积 _(1)(t)*(f)_(2)(t)-|||-(1) _(1)(t)=u(t), _(2)(t)=(e)^-at(t)-|||-(2) _(1)(t)=g(t) _(2)(t)=cos (omega t+(45)^circ )-|||-(3) _(1)(t)=(1+t)[ u(t)-u(t-1)] _(2)(t)=u(t-1)-u(t-2)-|||-(4) _(1)(t)=cos (omega t) _(2)(t)=s(t+1)-g(t-1)-|||-(5) _(1)(t)=(e)^-at(t) _(2)(t)=(sin t) a(t)