7a+3a=(7+3)a=
(2)-7a+3a=_______=_______
(3)-7a-3a=_______=_______
(4）若a与b互为倒数,则a+b=______,若A＜0,b＞0.
a
（5）若a与b互为倒数,则a*b=______.则ab_____0,----____0
b