已知a小于0,b大于0,且/a/大于/b/化简/a+b/+/a-b/-/-a-b/
a<0 b>0
|a|>|b|
|a+b|+|a-b|-|-a-b|
=(-a+b)+(-a-b)-(-a-b)
=-a+b-a-b+a+b
=-a+b