用 错位相减法求：1\2-1\4+1\8+1\32-1\64=1\128得和.
人气：455 ℃ 时间：2020-10-01 10:06:26
解答
另sn=1/2-1/4+1/8-1/16+1/32-1/64+1/128
1/2sn=1/4-1/8+1/16-1/32+1/64-1/128+1/256
3/2sn=1/2+1/256=129/256
sn=43/128
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