Câu 1.19 :
- $\left ( \frac{1}{9} \right )^{5} $ = $\left [\left ( \frac{1}{3} \right ) ^{2} \right ]^{5}$ = $\left ( \frac{1}{3} \right )^{10} $
- $\left ( \frac{1}{27} \right )^{7} $ = $\left [\left ( \frac{1}{3} \right ) ^{3} \right ]^{7}$ = $\left ( \frac{1}{3} \right )^{21} $