f(t)=f(t)*d(t)*u(t)=f(t)*(t) f(t)=f()*(1)*t(t)=f()求t() fi(t) 例题 1.P85.2-19(a) 2 f2(t) 2 2( ) ( ) ( ) ( ) ( ) ( ) ' ' f t  f t   t  u t  f t  u t ( ) ( ) ( ) ( ) ( ) ( ) '' '' f t  f t   t  tu t  f t  tu t 四.例题： 1.P85.2-19(a) 1 f1(t) 1 2 3 t f2(t) -2 2 t