Jumlah bilangan asli antara 1 sampai 30 yg habis dibagi 3

3, 6, 9, ...., 27

a = 3
b = 3

Un = a + (n - 1)b
27 = 3 + (n - 1)3
27 = 3n
n = 9

Sn = n/2 (a + Un)
S9 = 9/2 (3 + 27)
S9 = 9/2 × 30
S9 = 135