1990/1990x1991+1990/1991X1992.....+1990/1999X2000

日期：2013-04-26 22:09:30 人气：1

原式=1990/1990-1990/1991+1990/1991-1991/1992+……+1990/1999-1990/2000 =1-1990/2000 =1/200