k/n-1/n+1-1/n+2-.-1/n+k=(1/n-1/(n+1))+...+(1/n-1/(n+k))
而(1/n-1/(n+i))*n^2=i/(1+i/n).当n趋于无穷时极限为i
所以原式=1+2+.+k=k(k+1)/2