a) $(\dfrac{1}{3})^{n}=\dfrac{1}{81}$
$\Rightarrow(\dfrac{1}{3})^{n}=(\dfrac{1}{3})^4$
$\Rightarrow{n=4} ( thỏa \ mãn \ n∈Z )$
c) $(\dfrac{-3}{4})^{n}=\dfrac{81}{256}$
$\Rightarrow(\dfrac{-3}{4})^{n}=(\dfrac{-3}{4})^4 $
$\Rightarrow{n=4} ( thõa \ mãn \ n∈Z ) $