$\frac{x}{3}+\frac{15}{x}\geq 2\sqrt{\frac{x}{3}.\frac{15}{x}}$ $=2\sqrt{5}$ ( cauchy)Vay GTNN cua y la $2\sqrt{5}$ tai $x3\sqrt{5}$
$\frac{x}{3}+\frac{15}{x}\geq 2\sqrt{\frac{x}{3}.\frac{15}{x}}$ $=2\sqrt{5}$ ( cauchy)Vay GTNN cua y la $2\sqrt{5}$ tai $\pm3\sqrt{5}$
$\frac{x}{3}+\frac{15}{x}\geq 2\sqrt{\frac{x}{3}.\frac{15}{x}}$ $=2\sqrt{5}$ ( cauchy)Vay GTNN cua y la $2\sqrt{5}$ tai $
x3\sqrt{5}$