G.P.: (1,\ \dfrac{1}{3},\ \dfrac{1}{9},\ \ldots)

(a=1), (r=\dfrac{1}{3}), (n=15)

[ S_{15} = \dfrac{1-\left(\dfrac{1}{3}\right)^{15}}{1-\dfrac{1}{3}} = \dfrac{3}{2}\left(1-\dfrac{1}{3^{15}}\right) ]

Answer: (\dfrac{3}{2}\left(1-\dfrac{1}{3^{15}}\right))