menentukan jumlah suku ke n barisan geometri

[tex]S_n = \frac{a(r_{n}-1)}{r-1} = \frac{12( \frac{1}{2}^{n}-1)}{ \frac{1}{2} -1} = \frac{2^{2} x 3 (2^{-n}-1)}{-\frac{1}{2}} = -3 (2^{3-n} - 2^{3})[/tex]