Sn=4n+n(n-1)/2乘6=3n^2+n 怎么化简来的?
人气：429 ℃ 时间：2020-05-10 22:30:21
解答
Sn=4n+n(n-1)/2x6
=4n+3n^2-3n
=3n^2+n
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