1 3 6 10 15 Cari rumus rumu ke n?

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

1, 3, 6, 10, 15, ... → a = 1
..2..3..4....5... → b = 2
.....1...1.....1... → c = 1

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
Un = 1 + (n - 1) . 2 + 1/2 . (n² - 3n + 2) . 1
Un = 1 + 2n - 2 + 1/2 n² - 3n/2 + 1
Un = 1/2 n² + 1/2 n