3/4+15/28-1/4-1/28=（3/4-1/4）+（15/28-1/28）=1/2+1/2=1
=(3/4-1/4)+(15/28-1/28)=2/4+14/28=1/2+1/2=1
=3/4-1/4+（15/28-1/28）=2/4+14/28=1/2+1/2=1
