当a=-1,土时,有 =1-x+x2-x3+…+(-1)"x"+…(-1,1) 1·3 +x=1+-x 2 (2n-3)! r +.. x十 22.4 2.4.6 (2n)! 1·3 1.35 1--x+ x+…+(-1) (2n-1 y+∴ 1+x 22.4 2.4.6 (2n) 〈双阶乘>当 时, 有 2 1  = −1, 1 ( 1) ( 1,1) 1 1 2 3 = − + − + + − + − + x x x  n x n  x [ 1,1] (2 )!! (2 3)!! ( 1) 2 4 6 1 3 2 4 1 2 1 1 1 2 3 − + − + + −    +  + = + −  n x n  n n x x x x [ 1,1] (2 )!! (2 1)!! ( 1) 2 4 6 1 3 5 2 4 1 3 2 1 1 1 1 2 3 − + − + + −     −   = − + +  n x n  n n x x x x 双阶乘