Veja:
[tex3]S_{3n} = 39 + S_n \therefore \frac{(1 + 3n) \cdot 3n}{2} = 39 + \frac{(1+n) \cdot n}{2} \therefore 3n + 9n^2 = 78 + n + n^2 \therefore \\\\ 8n^2 + 2n - 78 = 0 \therefore 4n^2 + n - 39 = 0 \Leftrightarrow n_1 = \frac{-1 + 25}{8} \therefore n_1 = 3 \,\,, n_2 < 0 \\\\
S_{4n} = \frac{(1 + 4 \cdot n) \cdot 4n}{2} \therefore S_{4n} = \frac{13 \cdot 12}{2} \therefore S_{4n} = 78[/tex3]...