1: =x^2+6x+9+3
=(x+3)^2+3>=3
Dấu = xảy ra khi x=-3
2: =-(x^2-x-1)
=-(x^2-x+1/4-5/4)
=-(x-1/2)^2+5/4<=5/4
Dấu = xảy ra khi x=1/2
3: =4x^2+12x+9-7
=(2x+3)^2-7>=-7
Dấu = xảy ra khi x=-3/2
4: =-2(x^2-4x-2)
=-2(x^2-4x+4-6)
=-2(x-2)^2+12<=12
Dấu = xảy ra khi x=2