解：?$(1)$?∵點(diǎn)?$A，$??$B$?關(guān)于?$y$?軸對稱，∴橫坐標(biāo)互為相反數(shù)，縱坐標(biāo)相等
則?$-b=-2，$??$a=4，$?解得?$b=2，$??$a=4$?
?$ (2)$?點(diǎn)?$A，$??$C$?關(guān)于?$x$?軸對稱，∴?$C(2，$??$-a)；$?點(diǎn)?$B，$??$C$?關(guān)于?$y$?軸對稱，∴?$C(b，$??$4)$?
則?$2=b，$??$-a=4，$?解得?$a=-4，$??$b=2$?
