Đáp án:
$\dfrac{27}{343}$
Giải thích các bước giải:
$(\dfrac{3}{7})^{21}÷(\dfrac{9}{49})^9$
$⇒ (\dfrac{3}{7})^{21}÷[ (\dfrac{3}{7})^{2})^9]$
$⇒ (\dfrac{3}{7})^{21}÷ (\dfrac{3}{7})^{18}$
$⇒ (\dfrac{3}{7})^{21-18}$
$⇒ (\dfrac{3}{7})^{3}$
$⇒ \dfrac{27}{343}$