（+1.25）+（-1/2)+(-3/4)+(+1又3/4)=1.25-0.5-3/4+1+3/4=1.25+1-0.5-3/4+3/4=1.75
（+1.25）+（-1/2)+(-3/4)+(+1又3/4)=1.25-0.5-0.75+1.75=0+1.75=1.75
1.25-0.5-0.75+1.75=1.75
