a: =4(x^2-1/2x+3/4)
=4(x^2-2*x*1/4+1/16+11/16)
=4(x-1/4)^2+11/4>=11/4
Dấu = xảy ra khi x=1/4
b: =x^2+5x+25/4-53/4
=(x+5/2)^2-53/4>=-53/4
Dấu = xảy ra khi x=-5/2
c: =3(x^2-2/3x+1/3)
=3(x^2-2*x*1/3+1/9+2/9)
=3(x-1/3)^2+2/3>=2/3
Dấu = xảy ra khi x=1/3
d: =1/2(x^2+2x-6)
=1/2(x^2+2x+1-7)
=1/2(x+1)^2-7/2>=-7/2
Dấu = xảy ra khi x=-1