问题标题：
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=
问题描述：
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=
李雪斌回答：
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=（1-1/2）+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……+(1/9-1/10)
=1-1/10
=9/10
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