Question

banyak bilangan kelipatan 3 antara 20 dan 500?

Answer 1

21,24,27,30,33,36,39,42,45,,48,51,54,57,60,63,............500
jikakelipatan 3 antar 400,500
caranya 
Bilangan itu adalah : 402, 405, 408, ....., 498
a = 402, b = 3

Un = a + (n - 1)b
498 = 402 + (n - 1)3
498 = 402 + 3n - 3
498 = 399 + 3n
3n = 498 - 399
3n = 99
n= 33

Sn = 1/2 n (a + Un)
.....= 1/2 x 33 x (402 + 498)
.....= 16,5 x 900
Sn = 14.850