1/(1*4)+1/(4*7)+1/(7*10)+……+1/[(3n-2)(3n+1)]求和
题目
1/(1*4)+1/(4*7)+1/(7*10)+……+1/[(3n-2)(3n+1)]求和
答案
1/(1*4)+1/(4*7)+1/(7*10)+……+1/[(3n-2)(3n+1)]
=3(1-1/4+1/4-1/7+...+1/(3n-2)-1/(3n+1))
=3*(1-1/(3n+1))
=9n/3n+1
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