The value of

(a)

$\sqrt[4]{(81)^{-2}}=\sqrt[4]{\frac{1}{(81)^{2}}}=\frac{1}{(81)^{2 / 4}}=\frac{1}{(81)^{1 / 2}}$        $\left[\because \sqrt[m]{a^{n}}=a^{n / m}\right]$

$=\frac{1}{\left(9^{2}\right)^{1 / 2}}=\frac{1}{9^{2 / 2}}=\frac{1}{9}$                  $\left[\because\left(a^{m}\right)^{n}=a^{m n}\right]$