Find S12 for the series 1 + 1.5 + 2.25 + 3.375 +...

So we have

1((1.5)¹² -1) / (1.5 - 1) =

((1.5)¹² -1) / (.5 ) =

$$\mathrm{\ }$$$${\mathtt{2}}{\mathtt{\,\times\,}}\left({\left({\mathtt{1.5}}\right)}^{{\mathtt{12}}}{\mathtt{\,-\,}}{\mathtt{1}}\right) = {\mathtt{257.492\: \!675\: \!781\: \!25}}$$