（-0.8）+1.2+（-0.7）+（-2.1）+0.5+3.5=-0.8+1.2-0.7-2.1+0.5+3.5=(-0.8+1.2)-(0.7+2.1)+(0.5+3.5)=0.4-2.8+4=-2.4+4=1.62分之1+（-3分之2）+5分之4+（-2分之1）+（-3分之1）=1/2-2/3+4/5-1/2-1/3=(1/2-1/2)-(2/3+...