5/7x(2/7-1/2)【通分】= 5/7x(4/14-7/14)= 5/7x(-3/14)=-15/98
5/7(2/7-1/2)=5/7×(4/14-7/14)=5/7×(-3/14)=-15/98
横式计算是=5/7×(-3/14)=-15/98