（-0.8）+1.2+（-0.7）+（-2.1）+0.8+3.5
=(-0.8+0.8)+(1.2+3.5-0.7)-2.1
=4-2.1
=1.9
1/2+（-2/3）+4/5+（-1/2）+（-1/3）
=(1/2-1/2)+4/5-(2/3+1/3)
=4/5-1
=-1/5
-4.2+5.7-8.4+10
=(5.7+10)-(4.2+8.4)
=15.7-12.6
=3.1
-1/4+5/6+2/3-1/2
=(5/6+2/3)-(1/4+1/2)
=(5/6+4/6)-(1/4+2/4)
=9/6-3/4
=3/2-3/4
=6/4-3/4
=3/4