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Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

3, 9, 18, 30, ...→ a = 3
  6, 9,  12, ..  → b = 6
    3,  3, ..... → c = 3
n = 8

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U8 = 3 + 7 . 6 + 1/2 . 7 . 6 . 3
U8 = 3 + 42 + 63
U8 = 108 batang korek api