check the picture below.
now, let's rationalize the denominator on that.
[tex]\bf \cfrac{10\sqrt{5}}{\sqrt{2}}\implies \cfrac{10\sqrt{5}}{\sqrt{2}}\cdot \cfrac{\sqrt{2}}{\sqrt{2}}\implies \cfrac{10\sqrt{5}\cdot \sqrt{2}}{(\sqrt{2})^2}\implies \cfrac{10\sqrt{5\cdot 2}}{2}\implies 5\sqrt{10}[/tex]
