giúp với ạ

a) $\frac{-5\sqrt{x^2+1}}{2\sqrt3}=\frac{-5\sqrt{x^2+1}\times \sqrt3}{2\times (\sqrt3)^2}=\frac{-5\sqrt{3(x^2+1)}}{2\times 3}=\frac{-5\sqrt{3(x^2+1)}}{6}.$ b) $\frac{a^2-2a}{\sqrt a+\sqrt2}=\frac{a(a-2)}{\sqrt a+\sqrt2}=\frac{a(a-2)\times (\sqrt a-\sqrt2)}{(\sqrt a+\sqrt2)(\sqrt a-\sqrt2)}=\frac{a(a-2)\times (\sqrt a-\sqrt2)}{(\sqrt a)^2-(\sqrt2)^2}=\frac{a(a-2)\times (\sqrt a-\sqrt2)}{a-2}=a(\sqrt a-\sqrt2).$