1/2+1/6+1/12+1/20+.1/90

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1/2+1/6+1/12+1/20+.1/90
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1/2+1/6+1/12+1/20+.1/90
1/2+1/6+1/12+1/20+.1/90

1/2+1/6+1/12+1/20+.1/90
1/2+1/6+1/12+1/20+.1/90=1-1/2+1/2-1/3+……+1/9-1/10=1-1/10=9/10

1/2+1/6+1/12+1/20+......1/90
=1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10
=1/2+1/2-1/10
=9/10

很简单啊。。就是打起字来特麻烦，求给高分。。
1/1X2+1/2X3+1/3X4+1/4X5....+1/9X10
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/9-1/10)
=1-1/10
=9/10