jumlah bilangan kelipatan 3 antara 400 dan 500 = ....

402 , 405 , 408 , ... , 498
a = 402
b = 3
Un = 498
Sn?

Un = a + (n -1)b
498 = 402 + (n-1)3
498-402 = (n-1)3
96 = (n -1)3
96/3 = n-1
n = 33

Sn = n/2 (a + Un)
S33 = 33/2 (402 + 498)
S33 = 33/2 ( 900)
S33 = 14.850