1.7+（-2）-3.4
=7-2-3.4
=5-3.4
=1.6
2.-21.6+3-7.4+(-2/5)
=（-21.6-7.4）+3-2/5
=-29+3-2/5
=-26又五分之二
3.31+（-5/4）+0.25
=31-1.25+0.25
=31-1
=30
4.7-（-1/2）+1.5
=7+0.5+1.5
=7+2
=9
5.49-（-20.6）-3/5
=49+20.6-0.6
=49+20
=69
6.（-6/5）-7 -（-3.2）+(-1)
=-1.2-7+3.2-1
=（-1.2+3.2）-8
=2-8
=-6