$\large\text{Bạn tham khảo (:[tự làm]:)}$
$3A = 3 \times ( \dfrac{1}{3} + \dfrac{1}{9} + \dfrac{1}{27} + \dfrac{1}{81} + \dfrac{1}{243} + \dfrac{1}{729} + \dfrac{1}{2187})$
$3A = 1 + ( \dfrac{1}{3} + \dfrac{1}{9} + \dfrac{1}{27} + \dfrac{1}{81} + \dfrac{1}{243} + \dfrac{1}{729} )$
$3A = 1 + A - \dfrac{1}{2187}$
$2A = 1 - \dfrac{1}{2187}$
$2A = \dfrac{2186}{2187}$
$A = \dfrac{2186}{2187} \div 2$
$A = \dfrac{1093}{2187}$
