1/12x13 1/13x14 ... 1/19x20 请教解法，谢谢

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1/12x13 = 1/12 -1/131/13x14 = 1/13 -1/14余此类推。所以1/12 -1/13 + 1/13 - 1/14 + …… + 1/19 - 1/20 = 1/12 -1/20 = 1/30