Câu 3:
a, $\sqrt{\dfrac{2}{5}}=\dfrac{\sqrt2}{\sqrt5}=\dfrac{\sqrt{10}}{5}$
b, $\sqrt{\dfrac{5}{18}}=\dfrac{\sqrt5}{3\sqrt2}=\dfrac{\sqrt{10}}{6}$
c, $\sqrt{\dfrac{4}{27}}=\dfrac{2}{3\sqrt3}=\dfrac{2\sqrt3}{9}$
d, $\sqrt{\dfrac{x}{2y}}=\dfrac{\sqrt{x}}{\sqrt{2y}}=\dfrac{\sqrt{2xy}}{2y}$