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從1,2,3,4,…,20這20個自然數中任取3個不同的數,使它們成等差數列,這樣的等差數列共有幾個【解】公差為1的:(1,2,3),(2,3,4),…,(18,19,20)共18個;公差為2的:(1,3,5),(2,4,6),…,(16,18,20)共16個;公差為3的:…
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