Q.Find $S_n$ of the following arithmetico-geometric sequence: $3, 12, 36, 96, 240, \ldots$

An Arithmetico-Geometric Progression (A.G.P.) is a sequence in which each term is the product of the corresponding terms of an A.P. and a G.P.: if the A.P. is $a, a+d, a+2d, \ldots$ and the G.P. is $1, r, r^2, \ldots$, the A.G.P. is $a, (a+d)r, (a+2d)r^2, \ldots$, so its general term is $t_n=[a+(n-1)d]\,r^{n-1}$. The sum of the first $n$ terms is found by the same shift-and-subtract trick used for a plain G.P.: write $S_n$, then write $rS_n$ (every term shifted one place), and subtract; because the A.P. part now differs by a constant $d$, the leftover terms form a genuine G.P. that can be summed …