1999又1999分之1998除以1998
= (1999 + 1998/1999) / 1998
= 1999/1998 + 1/1999
= 1+ 1/1998 + 1/1999
= 1.0010007506255630317661333169087
你可近具体要求取舍小数位数用分数啊