tolong dibantu ya..beserta cranya trimakasih

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

1, 3, 6, 10, ... → a = 1
..2..3...4.... → b = 2
.....1...1....→ c = 1

n = 30

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c

U30 = 1 + 29 . 2 + 1/2 . 29 . 28 . 1
U30 = 1 + 58 + 29 . 14
U30 = 59 + 406
U30 = 465

✩✩ D ✩✩

kalau menurut aku begini
n(n+1)/2
30(30+1)/2 = 465