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Kelas 8 MatematikaBab Barisan dan Deret Bilangan
1, 3, 6, 10, .... → a = 1
..2..3..4...... → b = 2
.....1....1... → c = 1
n = 15
Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U15 = 1 + 14 . 2 + 1/2 . 14 . 13 . 1
U15 = 1 + 28 + 91
U15 = 120
✩✩ C ✩✩