(5a+4)+(2a-3)
=5a+4+2a-3
=7a+1
(7x+3)-(-2x-5)
=7x+3+2x+5
=9x+8
3a-(a-5)
=3a-a+5
=2a+5
都是狠简单的拆括号~不是苯`` 是懒``
认真学吧~!