a) $A=x^2-3x+24$
$=x^2-2.x.\dfrac{3}{2}+\dfrac{9}{4}+\dfrac{87}{4}$
$=(x-\dfrac{3}{2})^2+\dfrac{87}{4}$
Ta thấy: $(x-\dfrac{3}{2})^2≥0$
$→$ Dấu "=" xảy ra khi $x-\dfrac{3}{2}=0$
$→x=\dfrac{3}{2}$
$→A_{min}=\dfrac{3}{2}$
b) $B=x^2+16x+70$
$=x^2+2.x.8+64+6$
$=(x+8)^2+6$
Ta thấy: $(x+8)^2≥0$
$→$ Dấu "=" xảy ra khi $x+8=0$
$→x=-8$
$→B_{min}=6$
c) $C=x^2-5x+1$
$=x^2-2.x.\dfrac{5}{2}+\dfrac{25}{4}-\dfrac{21}{4}$
$=(x-\dfrac{5}{2})^2-\dfrac{21}{4}$
Ta thấy: $(x-\dfrac{5}{2})^2≥0$
$→$ Dấu "=" xảy ra khi $x-\dfrac{5}{2}=0$
$→x=\dfrac{5}{2}$
$→C_{max}=-\dfrac{21}{4}$
