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巧算:1/1×2+1/2×3+1/3×4+.+1/19×20+1/20×21
人气:261 ℃ 时间:2019-12-11 18:07:39
解答
1/1×2+1/2×3+1/3×4+.+1/19×20+1/20×21
=1-1/2+1/2-1/3+1/3-1/4+……+1/20-1/21
=1-1/21
=20/21
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