The sequence $(a_n)_{n \ge 1}$ starts $-1, 0, 2, 5, 9, 14\ldots$ and has closed formula $a_n = \dfrac{(n+1)(n-2)}{2}\text{.}$ Use this fact to find a closed formula for the sequence $(b_n)_{n \ge 1}$ which starts $4, 10, 18, 28, 40, \ldots\text{.}$ $b_n =$