$a)D=3x^2-8x+7$
$D=3(x^2-\dfrac{8}{3}x+\dfrac{7}{3})$
$D=3(x^2-2.x.\dfrac{4}{3}+\dfrac{16}{9}+\dfrac{5}{9})$
$D=3(x-\dfrac{4}{3})^2+\dfrac{5}{3}\ge\dfrac{5}{3}$
Dấu $"="$ xảy ra
`<=>`$x-\dfrac{4}{3}=0$
`<=>`$x=\dfrac{4}{3}$
Vậy $MinD=\dfrac{5}{3}$ khi $x=\dfrac{4}{3}$
$e)E=x^4-2x^2+12$
$E=(x^2)^2-2.x^2.1+1+11$
$E=(x^2-1)^2+11\ge11$
Dấu $"="$ xảy ra
`<=>`$x^2-1=0$
`<=>`$x^2=1$
`<=>`$x=\pm1$
Vậy $MinE=11$ khi $x=\pm1$
