用数学归纳法证明当n=1时 左边=1/2+1/3+1/4=13/12>1,成立假设n=k时成立 即1/(k+1)+1/(k+2)+1/(k+3)...+1/(3k+1)>1当n=k+1时 即要证明 1/(k+2)+1/(k+3)+...+1/(3k+1)+1/(3k+2)+1/(3k+3)+1/(3k+4)>1式子里比n=k的式子...