左边=1/[4n(n+1)]
=1/4*1/n(n+1)
=1/4*[(n+1)-n]/n(n+1)
=1/4*[(n+1)/n(n+1)-n/n(n+1)]
=1/4*[1/n-1/(n+1)]
为什么1/2n(2n+2)=1/4(1/n-1/n+1)
2个回答
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