An arithmetic-geometric series has terms: a, (a+d)r, (a+2d)$r^2$, ..., [a+(n-1)d]r^(n-1). Sum technique: Let S = sum. Compute rS. Then S - rS = a + d(r+$r^{2+}$...+r^(n-1)) - [a+(n-1)d]$r^n$. Simplify the GP part. For infinite series (|r|<1): S = $\frac{a}{1-r}$ + $\frac{dr}{1-r}$^2.