8分之1+7分之1+6分之1+.+二分之一
=(1/8+1/4+1/2)+(1/6+1/3)+1/5+1/7
=7/8+3/6+1/5+1/7
=(7/8+1/2)+(1/5+1/7)
=11/8+12/35
=481/280