Pembahasan:
Jumlah bilangan antara 300 dan 500 yang habis dibagi 6:
306 + 312 + 318 + . . . + 486 = . . . .
a = 306, b = 6, dan Uₙ = 486
Uₙ = a + (n – 1)b = 486
306 + [(n – 1) × 6] = 486 – 306
(n – 1) × 6 = 486 – 306
(n – 1) × 6 = 180
n – 1 = 180 ÷ 6
n – 1 = 3
n = 31
Sₙ = n(a + Uₙ)
2
S₃₁ = 31 (306 + 486)
2
= 31 × 792
2
= 12.276