②.可用母函数法,比较简便 证明:(1)4(*) x-t ∑P(x) 2xt+1 两边都乘上(-2x+),并再用() (x-)∑2(x)=(2x+2)∑B(x)②.可用母函数法，比较简便 证明：(1) ( ) ( ) -1 3 2 0 2 - 1- 2 l l l x t P x lt xt t ¥ = = å + ( ) 2 两边都乘上 1- 2xt t + ，并再用(*) ( ) ( ) ( ) ( ) 2 -1 0 0 - 1- 2 l l l l l l x t P x t xt t lP x t ¥ ¥ = = å å = + d dt (*)