3/4+15/28-1/4-1/28=21/28+15/28-7/28-1/28=(21+15-7-1)/28=28/28=1仅供参考，祝您学习进步！
=3/4-1/4 +15/28-1/28=(3-1)/4 +(15-1)/28=2/4+14/28=1/2+1/2=1
3/4+15/28-1/4-1/28=1/2+1/2=1
