a. $\sqrt{\dfrac{2}{7}} = \sqrt{\dfrac{2.7}{7^2}} = \sqrt{\dfrac{14}{7^2}} = \dfrac{1}{7}\sqrt{14}$
b. $\sqrt{\dfrac{x^2}{31}} = \sqrt{\dfrac{x^2.31}{31^2}} = \dfrac{|x|}{31}\sqrt{31}$
$= \dfrac{x}{31}\sqrt{31}$ với $x \geq 0$
hoặc $= \dfrac{-x}{31}\sqrt{31}$ với $x < 0$