tolong dibantu yaa teman

12, 20, 30, 42, 56, .... → a = 12
....8.....10...12....14.... → b = 8
.......2......2......2.....→ c = 2

n = 22

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U22 = 12 + 21 . 8 + 1/2 . 21. 20 . 2
U22 = 12 + 168 + 420
U22 = 600