数列求和习题1/(1+2)+1/(1+2+3)+~+1/(1+2+3+~+n+1)
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数列求和习题1/(1+2)+1/(1+2+3)+~+1/(1+2+3+~+n+1)
![数列求和习题1/(1+2)+1/(1+2+3)+~+1/(1+2+3+~+n+1)](/uploads/image/z/2468595-3-5.jpg?t=%E6%95%B0%E5%88%97%E6%B1%82%E5%92%8C%E4%B9%A0%E9%A2%981%2F%281%2B2%29%2B1%2F%281%2B2%2B3%29%2B%7E%2B1%2F%281%2B2%2B3%2B%7E%2Bn%2B1%29)
1/(1+2+3+...+n)=2/[(n+1)*n]=2*(1/n-1/(n+1));
所以1/(1+2)+1/(1+2+3)+……+1/【1+2+3+…….+(n+1)】=2(1/2-1/3+1/3-1/4+...+1/(n+1)-1/(n+2))=2*(1/2-1/(n+2))=1-2/(n+2)=n/(n+2)
所以1/(1+2)+1/(1+2+3)+……+1/【1+2+3+…….+(n+1)】=2(1/2-1/3+1/3-1/4+...+1/(n+1)-1/(n+2))=2*(1/2-1/(n+2))=1-2/(n+2)=n/(n+2)