in short, you use 1, since any number is itself times 1, or whatever = whatever * 1
\(\bf \cfrac{4}{\sqrt{50}}\iff \cfrac{4}{\sqrt{50}}\cdot 1\iff \cfrac{4}{\sqrt{50}}\cdot \cfrac{1}{1}\implies \cfrac{4}{\sqrt{50}}\cdot \cfrac{\sqrt{50}}{\sqrt{50}}
\\\\\\
\cfrac{4\sqrt{50}}{\sqrt{50^2}}\implies \cfrac{4\sqrt{50}}{50}\)
