recursive formula
http://prntscr.com/l2cxl6
\(t_1 = 1\\ t_{n+1} = t_n + n \\ t_2 = 1 + 2 = 3\\ t_3 = 3 + 3 = 6 \\ t_4 = 6 + 4 = 10 \\ t_5 = 10 + 5 = 15\)
This matches up with choice (c)
