Cấp số nhân có: \({u_1}=1 \), \(\displaystyle q = {1 \over 3}\)
\( \displaystyle \Rightarrow S = {{{u_1}(1 - {q^n})} \over {1 - q}} = {{1.\left[ {1 - {{({1 \over 3})}^n}} \right]} \over {1 - {1 \over 3}}} \) \(\displaystyle = {2 \over 3}\left[ {1 - {{({1 \over 3})}^n}} \right]\)
