解:?$(1)$?當(dāng)?$a = 2,$??$b = 6$?時(shí),點(diǎn)?$C$?所表示的數(shù)為?$(2 + 6)\div 2 = 4$?
?$ $?當(dāng)?$a = 0,$??$b = 6$?時(shí),點(diǎn)?$C$?所表示的數(shù)為?$(0 + 6)\div 2 = 3$?
?$ $?當(dāng)?$a = 2,$??$b = - 6$?時(shí),點(diǎn)?$C$?所表示的數(shù)為?$(2+(-6))\div 2=-2$?
?$ $?當(dāng)?$a=-2,$??$b = - 6$?時(shí),點(diǎn)?$C$?所表示的數(shù)為?$(-2+(-6))\div 2=-4$?
?$ (2)x$?與?$a,$??$b$?之間的關(guān)系為?$x=(a + b)\div 2$?