1/2-3又1/4+2.75-7又1/2=(1/2-7又1/2)+(2.75-3.25)=(-7)+(-0.5)=-7.5
=0.5+3.25+2.75-7.5=6.5-7.5=-1
1/2-(-3又1/4)+2.75-7又1/2=1/2+13/4+11/4-7又1/2=1/2-7又1/2+(13/4+11/4)=-7+6=-1
