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