B = 4x - 2√x - 5 Đkxđ: x ≥ 0
= 4x - 2√x + $\frac{1}{4}$ - $\frac{21}{4}$
= (2√x - $\frac{1}{2}$)² - $\frac{21}{4}$
Có: (2√x - $\frac{1}{2}$)² ≥ 0 với mọi x ≥ 0
⇒ (2√x - $\frac{1}{2}$)² - $\frac{21}{4}$ ≥ - $\frac{21}{4}$
⇒ B ≥ - $\frac{21}{4}$
Dấu "=" xảy ra ⇔ 2√x - $\frac{1}{2}$ = 0
⇔ 2√x = $\frac{1}{2}$
⇔ √x = $\frac{1}{4}$
⇔ x = $\frac{1}{16}$ (t/m)
Vậy min B = - $\frac{21}{4}$ khi x = $\frac{1}{16}$
