y = 1 + [x/(x+1)(x-2)]
= 1 + (x-2)^(-1) - [1/(x+1)(x-2)]
= 1 + (x-2)^(-1) - (1/3)[1/(x-2)] + (1/3)[1/(x+1)]
= 1 + (2/3)·(x-2)^(-1) + (1/3)·(x+1)^(-1)
∴y的n阶导数 = (2/3)·(-1)^n·n!·(x-2)^(-n-1) + (1/3)·(-1)^n·n!·(x+1)^(-n-1)
= [(-1)^n·n!/ 3]·[2·(x-2)^(-n-1) + (x+1)^(-n-1)]