Câu 4:
\[A=2(x+3)^2-5\] Vì $2(x+3)^2\ge 0\to A\ge -5$ $\to Min_A=-5$ khi $x=-3$
Câu 9:
$a)$
Xét $ΔMKP$ và $ΔMKQ$ có:
$MK\quad chung$
$KP=KQ(gt)$
$MP=MQ(gt)$
$→ΔMKP=ΔMKQ$
$b)$ $ΔMKP=ΔMKQ→\widehat{MKP}=\widehat{MKQ}$
mà $\widehat{MKP}+\widehat{MKQ}=\widehat{PKQ}=180^{\circ}$
$→\widehat{MKP}=\widehat{MKQ}=90^{\circ}$
$→MK\quad \bot \quad PQ$