=1/1996-1/1997+1/1997-1/1998+1/1998-1/1999+1/1999 =1/1996 一般规律：1/[n(n+1)]=1/n-1/(n+1)
1997+1996x1998=1998-1+1998*1996=1998*1997-1以后类似
