（-0.5）+3又4分之1+2.75+（-5又2分之1）=（-0.5-5.5）+（3.25+2.75）=-6+6=0
（-0.5）+3（1/4）+2.75+（-5（1/2））=（-0.5）+3.25+2.75+（-5.5）=-（0.5+5.5）+（3.25+2.75）=-6+6=0
0,-0.5+3+0.25+2+0.75-5-0.5=0
