1/1987*1988+1/1988*1989+1/1989*1990+1/1990
=1/1987*1988+1/1988*1989+1/1989*1990+1989/1989*1990
(后两个相加)
=1/1987*1988+1/1988*1989+（1+1989）/1989*1990
=1/1987*1988+1/1988*1989+1/1989
=1/1987*1988+1/1988*1989+1988/1988*1989
(后两个相加)
=1/1987*1988+（1+1988）/1988*1989
=1/1987*1988+1/1988
=1/1987*1988+1987/1987*1988
=(1+1987)/1987*1988
=1/1987
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