1 + 1/3 + 1/9+1/27+...+1/729

=1+1-1/3+1/3-1/9+1/9-1/27-1/27...-1/243

= 2 - 1/243

=485/243

đặt S=1/3+1/9+1/27+1/81+1/243+1/729

=>3S= 1+1/3+1/9+1/27+1/81+1/243

=>3S-S=(1+1/3+...+1/243)−(1/3+1/9+...+1/729)

=>s=1-1/729
= 728/729
chả bt đúng k nếu sai thì sory bạn nhé