Câu 11: $B(x+3)(3-x)-(x-1)^2$
$B=(3+x)(3-x)-(x^2-2x+1)$
$B=9-x^2-x^2+2x-1$
$B=8-2x^2+2x \to$ Đáp án $B$.
Câu 12: $Q=8+12x-9x^2$
$Q=(-9x^2+12x-4)+12$
$Q=-(3x-2)^2+12$
Vì $-(3x-2)^2 \le 0 \;\forall x\in \mathbb{R}$
$→ -(3x-2)^2+12 \le 12\;\forall x\in \mathbb{R}$
Vậy $\max = 12$ khi $3x-2=0⇔x=\dfrac23 \to$ Đáp án $D$.
Câu 13: $x^3-3x^2+3x$
$= x^3-3x^2+3x-1+1$
$= (x-1)^3+1 = 10^3+1 = 1001 \to$ Đáp án $D$
