（6/1+1/7+1/8）*(1/7+1/8+1/9)-(1/6+1/7+1/8+1/9)*(1/7+1/8)假设：1/7+1/8=a,1/7+1/8+1/9=b,则：上式=（1/6+a)*b-(1/6+b)*a =1/6*b+ab-1/6a-ab =1/6(b-a) =1/6*1/9 =1/54
