Answer
$\dfrac {1}{2}$
Work Step by Step
$\sum ^{\infty }_{n=1}3^{-k}=\dfrac {a_{1}}{1-r};a_{1}=3^{-1}=\dfrac {1}{3};r=\dfrac {1}{3}\Rightarrow S_{\infty }=\dfrac {a_{1}}{1-r}=\dfrac {\dfrac {1}{3}}{1-\dfrac {1}{3}}=\dfrac {1}{2}$
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