1/(4×5)+1/(5x6)+1/(6x7)……1/(39x40) 有什么简便算法 .
2个回答
1/(4x5)=1/4-1/5
同理1/x(x+1)=1/x-1/(x+1)
所以该算式简化
1/4-1/40=9/40
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