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Sn = 1/2.n (a + Un)
1.218 = 1/2.n (8 + 5n + 3)
1.218 = 4n + 5/2.n + 3/2
2436 = 8n +5n² + 3
5n² + 8n - 2433 = 0
n = (-8 + √(64 + 48660)) / 10
= 21,27
Dalam hal ini n tidak mungkin pecahan atau merupakan bilangan negatif.