[(-4)-(+7)]-(-5)

=(-4-7)+5

=-11+5

=-6

2.  3-[(-3)-(+12)]

=3-(-3-12)

=3-(-15)

=3+15

=18

3.  数轴上a与b两点表示的有理数分别是负六右二分之一和七右四分之三,求两点间的距离

(7+3/4)-[-(6+1/2)]

=7+3/4+6+1/2

=13+3/4+2/4

=13+5/4

=14+1/4

两点间的距离：14又4分之1