1) $\sqrt{\dfrac{2}{7}} = \sqrt{\dfrac{2.7}{7.7}} = \sqrt{\dfrac{14}{7^2}} = \dfrac{1}{7}\sqrt{14}$
2) $\sqrt{\dfrac{x^2}{20}} = \sqrt{\dfrac{x^2.20}{20^2}} = \dfrac{|x}{20}\sqrt{20} = \dfrac{x}{20}.\sqrt{4.5} = \dfrac{2x}{20}\sqrt{5} = \dfrac{x}{10}\sqrt{5}$
3) $\sqrt{\dfrac{5x^3}{8y}} = \sqrt{\dfrac{5x^3.8y}{(8y)^2}} = \dfrac{x}{8y}\sqrt{40xy} = \dfrac{x}{4y}\sqrt{10xy}$
(Với $x \geq 0$; $y > 0$)
4) Đề không rõ ràng