A= 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/2187

$\large\text{Bạn tham khảo (:[tự làm]:)}$

$3A = 3 \times  ( \dfrac{1}{3} + \dfrac{1}{9} + \dfrac{1}{27} + \dfrac{1}{81} + \dfrac{1}{243} + \dfrac{1}{729} + \dfrac{1}{2187})$

$3A = 1 + ( \dfrac{1}{3} + \dfrac{1}{9} + \dfrac{1}{27} + \dfrac{1}{81} + \dfrac{1}{243} + \dfrac{1}{729} )$

$3A = 1 + A - \dfrac{1}{2187}$

$2A = 1 - \dfrac{1}{2187}$

$2A = \dfrac{2186}{2187}$

$A = \dfrac{2186}{2187} \div 2$

$A = \dfrac{1093}{2187}$