无穷级数1/1·6+1/6·11+…+1/[(5n-4)(5n+1)]+…的和为____________

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问题 无穷级数1/1·6+1/6·11+…+1/[(5n-4)(5n+1)]+…的和为____________

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答案1/5

解析 Sn=1/1·6+1/6·11+…+1/(5n-4)(5n+1)=1/5(1-1/6+1/6-1/11+…+1/5n―4―1/5n+1)=1/5(1-1/5n+1). 1/5(1-1/5n+1)=1/5.故所求无穷级数的和为1/5.
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