求Sn=1/1x2+1/2x3+1/3x4+.+1/n(n+1)
2个回答
裂项法:
Sn=1/1x2+1/2x3+1/3x4+.+1/n(n+1)
=1-1/2+1/2-1/3+1/3-1/4+……+1/n-1/(n+1)
=1-1/(n+1)
=n/(n+1)
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