$\begin{array}{l}
\sqrt {49} = \sqrt {{7^2}} = 7\,\,;\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\sqrt {\dfrac{4}{{81}}} = \sqrt {{{\left( {\dfrac{2}{9}} \right)}^2}} = \dfrac{2}{9}\\
\sqrt {25} = \sqrt {{5^2}} = 5\,\,;\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\sqrt {81} = \sqrt {{9^2}} = 9
\end{array}$