原式=1*1/2+1/2*1/3+1/3*1/4+…+1/9*1/10
=1-1/2+1/2-1/3+…+1/9-1/10
=1-1/10=9/10
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90 这道数学题的简便运算怎么解
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1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=
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2/1＋6/1＋12/1＋20/1＋30/1＋42/1＋56/1＋72/1＋90/1