=(-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