nomor 8 gimana caranya????

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

1, 3, 6, 10, .... → a = 1
..2..3..4...... → b = 2
.....1....1... → c = 1
n = 15

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U15 = 1 + 14 . 2 + 1/2 . 14 . 13 . 1
U15 = 1 + 28 + 91
U15 = 120

✩✩ C ✩✩