[tex]a_n = 4n - 10 \\ a_1 = 4 - 10 = -6 \\ a_2 = 8- 10 = -2 \\ a_3 = 12 - 10 = 2 \\ a_18 = 72 - 10 = 62 [/tex]
a = -6
d = 4
[tex]a_n = a + (n - 1)d \\ 62 = -6 + (n - 1)4 \\ 68/4 = n - 1 \\ n = 17 + 1 \\ n = 18 [/tex]
[tex]S_n = n/2 × (a + a_n) \\ S_n = 18/2 × ( -6 + 62) \\ S_n = 9 × 56 = 504[/tex]
