2 例5求∑的和 n=」 解令(x)=∑x (n 十n /(n ∵S(x)= ∑ 1) x"=∑ ∑ -=1 ∑”+x∑ =xe 1) e e(x+1)x, ∑ n 13 H-=1 !2 S()=e2(+1)例 5 . 1 !22 求  的和 n= n nn 解 , ! ( ) 1 2 n n x nn s x = 令 = (− ,+ ) n n x n n n n s x = − + = 1 ! ( 1)  ( ) n n n n x n x n n n  = = − + − = 1 1 ( 1)! 1 ! ( 1)  = = =  + 1 0 2 ! )! ( n n n n nx x nx x x x = x (e −1) + xe 2 e (x 1)x, x = + =  1 2 n !2n nn ) 21 = s( 21 1) 21( 21 = e + . 43 = e