1/21/3+2/3=11/5+2/5+3/5+4/5=21/7+2/7.+5/7+6/7=31/11+2/11+.+9/11+10/11=51/13+2/13+.+11/13+12/13=6.1/n+2/n+.(n-2)/n+(n-1)/n=(n-1)/2 (n为质数)1/17+.=81/19+.=91/2+1+2+3+5+6+8+9=69/2=34.5