in general

$\dfrac 1 {\dfrac a b}=\dfrac b a$

applied here we get

$\dfrac a {\dfrac{\sqrt{40}} 2}=a \cdot \dfrac 2 {\sqrt{40}}=\dfrac{2a}{\sqrt{4\cdot 10}}=\dfrac{2a}{2\sqrt{10}}

=\dfrac a {\sqrt{10}}=\dfrac a {\sqrt{10}}\dfrac{\sqrt{10}}{\sqrt{10}}=\dfrac {\sqrt{10}a}{10}$