这种求和方式较为少见，平时数学学习习中较少用到，但它确实非常巧妙。这里用到了一个方法，叫做构造2S，因为先把原来的数列整体看作为字母S，那么2S减去S正好仍然等于原来的数列。由于2S与S这两个数列中有重叠的部分，所以在相减的过程中重叠的部分全部消去，只剩下2S中的最后一个数减S中的第一个数，这样就非常方便地算出了得数，可以说巧妙至极。

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