1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72=(1/2+1/6+1/12+1/72）+（1/20+1/30）+（1/42+1/56）=（36+12+6+1）/72+（3+2）/60+（8+6）/336=55/72+1/12+1/24=（55+6+3）/72=64/72=8/9
