=6/7*5/6-6/7*1/2+1/2=5/7+1/2*(1-6/7)=5/7+1/2*6/7=5/7+3/7=8/7
6/7*(5/6-1/2)+1/2=6/7×5/6-6/7×1/2+1/2=5/7-3/7+1/2=2/7+1/2=11/14