[tex]\displaystyle\bf \bigg(\frac{625}{81} \bigg)^{-\frac{1}{2} }=\bigg(\frac{25}{9} \bigg)^{-2\cdot\frac{1}{2} }=\bigg(\frac{25}{9} \bigg)^{-1}=\boxed{\frac{9}{25} }[/tex]
