1/(n+1)+1/(n+2)+1/(n+3)+...1/2n＝1/n×[1/(1+1/n)+1/(1+2/n)+1/(1+3/n)+.+1/(1+n/n)]＝∑1/n×[1/(1+k/n)]k=1,2,.,n＝∑1/n×[1/(1+k/n)]n→+∞时k/n=0,.,1；dx=1/nf(x)=dx×[1/...