Ta có :'

• $3\sqrt{5}=\sqrt{3^{2}.5}=\sqrt{45}$
• $-5\sqrt{2}=-\sqrt{5^{2}.2}=-\sqrt{50}$
• $\frac{-2}{3}\sqrt{xy}=-\sqrt{(\frac{2}{3})^{2}xy}=-\sqrt{\frac{4}{9}xy}(xy\geq 0)$
• $x\sqrt{\frac{2}{x}}=\sqrt{x^{2}.\frac{2}{x}}=\sqrt{2x}(x>0)$