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Kelas 7 Matematika
Bab Pola Bilangan

1, 3, 6, 10, ... → a = 1
..2..3...4.... → b = 2
....1....1... → c = 1

n = 20

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U20 = 1 + 19 . 2 + 1/2 . 19 . 18 . 1
U20 = 1 + 38 + 171
U20 = 210

✩✩ C ✩✩