2 例5求∑ 的和 nI2 nE 解令(x)=∑,x 0,+0 ∵S(x)= 这mn-D、;′之n上” In n H-=1 "+x∑ =x2(e-1)"+xex H=0 =e(x+1)x ∑ n 3 H-=1 1n2 =S(7)=e2(+1) 22 上页例 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