`(d): x-2y-2=0`

Vì `M` $\in$ `(d)`

`-> M(2a+2; a)`

`-> MA^2=[1-(2a+2)]^2+(4-a)^2`

`=(2a-1)^2+(a-4)^2`

`=4a^2-4a+1+a^2-8a+16`

`=5a^2-12a+17`

`MB^2=[-1-(2a+2)]^2+(2-a)^2`

`=(2a+3)^2+(a-2)^2`

`=4a^2+12a+9+a^2-4a+4`

`=5a^2+8a+13`

`-> MA^2-2MB^2=5a^2-12a+17-10a^2-16a-26`

`=-5a^2-28a-9`

`=-5(a^2+(28)/(5)+(9)/(5))`

`=-5(a+(14)/(5))^2+(151)/(5)<=(151)/(5)`

Dấu `=` xảy ra khi `a=(-14)/(5)`

`-> M((-18)/(5); (-14)/(5))`