如图所示
解 见下图
12-3/4-1/4=12-(3/4+1/4)=12-1=11
12-3/4-1/4=12-(3/4+1/4)=12-1=112/3-1/4+3/8=2/3+(3/8-1/4)=2/3+1/8=19/24