Get an extra mark: 2[3^(n-1) + 3] :wink:soroban said:. . \(\displaystyle \text{we get: }\:\boxed{a_n \;=\;2\cdot3^{n-1} + 6}\\)
Use this to impress/surprise/terrify your teacher . . .
Subhotosh Khan said:\(\displaystyle \frac{4}{3}n^4 \, - \, \frac{32}{3}n^3 \, + \, \frac{104}{3}n^2 \, - \, \frac{136}{3}n \, + \, 28\)
But I like Soroban's expression better - it has less constants.