1/√n=2/(√n+√n)＞2/(√n+1+√n)=2(√n+1 -√n)所以1+1/√2+1/√3+...+1/√n＞2(√2-1)+2(√3-√2)+2(√4-√3)+...+2(√n+1-√n)=2(√n+1-1)右边也一样,1/√n=2/(√n+√n)＜2/(√n-1+√n)=2(√n -√n-1).....