左边：1/(n+1)+1/(n+2)+......+1/2n≥1/2n+1/2n+......+1/2n=n/2n=1/2右边:1/(n+1)+1/(n+2)+......+1/2n≤1/(n+1)+1/(n+1)+....1/(n+1)=n/(n+1)＜1 (左边就是把分母都换成2n，缩小,而右边是把分母换成n+1,放大，这就是所谓的放缩法)
