3)3<t<4如图284(c) fi(t) t-4 图2.8-4(c) f1(1)*/2(1)=后(t-)d 983 8 9 4 3 ( ) 4 1 ( ) ( ) 3 0 1  2 = − = −  f t f t t  d t 3) 3 t  4 ( ) 1 f  1 0  t t-4 如图2.8-4(c) 图2.8-4(c)