Consider the series
$s=(1-t)S \sum_{i=0}^k(1+r)^{k-i}$, where $S>0, 0
$$\sum_{i=0}^k(1+r)^{k-i}=\sum_{i=0}^k(1+r)^i=\frac{1-(1+r)^{k+1}}{1-(1+r)}=\ldots$$