（1）3与-2.2
=|3-(-2.2)|
=|3+2.2|
=5.2
（2）4.75与2.25
|4.75-2.25|
=|25.5|
=2.5
（3）-4与-4.5
|(-4)-(-4.5)|
=|-4+4.5|
=0.5
（4）-3又2/3与+2又1/3
|(-3又2/3)-(+2又1/3)|
=|-6|
=6