a,
A = x ( x - 1 )
⇒ A = x² - x
⇒ A = x² - 2 . $\frac{1}{2}$x + $\frac{1}{4}$ - $\frac{1}{4}$
⇒ A = ( x - $\frac{1}{2}$)² - $\frac{1}{4}$ ≤ - $\frac{1}{4}$
Dấu "=" xảy ra ⇔ x = $\frac{1}{2}$
Vậy Min A = 1/4 ⇔ x = $\frac{1}{2}$
B = 3x² + x + 5
⇔ B = 3 ( x² + $\frac{1}{3}$x + $\frac{5}{3}$ )
⇔ B = 3 ( x² + 2 . $\frac{1}{6}$x + $\frac{1}{36}$ + $\frac{59}{36}$ )
⇔ B = 3 ( x + $\frac{1}{6}$ )² + $\frac{59}{12}$ ≤ $\frac{59}{12}$
Dấu "=" xảy ra ⇔ x = - $\frac{1}{6}$
C = ( x + 1 )( x - 3 )
⇔ C = x² + x - 3x - 3
⇔ C = x² - 2x + 1 - 4
⇔ C = ( x - 1 )² - 4 ≤ - 4
Dấu "=" xảy ra ⇔ x = 1
