Giải thích các bước giải:
$A=x^2-4x+1=(x-2)^2-3$
$Amin=-3\Leftrightarrow x-2=0\Rightarrow x=2$
$B=4x^2+4x+11=(2x+1)^2+10$
$Bmin=10\Leftrightarrow 2x+1=0\Rightarrow x=-\dfrac12$
$C=3x^2-6x+1=3(x-1)^2-2$
$Cmin=-2\Leftrightarrow x-1=0\Rightarrow x=1$
$D=2+x-x^2=-(x-\dfrac12)^2+\dfrac94$
$Dmax=\dfrac94\Leftrightarrow x-\dfrac12=0\Rightarrow x=\dfrac12$