Jika Diketahui a=15, b=(-3), Sn=42. Maka tentukan nilai n!

Sn = n/2 (2a + (n-1)b)
42 = n/2 (2.15 + (n-1)-3)
42 = n/2 (30-3n +3)
42 = n/2 (-3n+33)
84 = n (-3n+33)
84 = -3n²+33n
3n²-33n+84 = 0
n²-11n+28 = 0
(n-7)(n-4) = 0
n = 7 atau n = 4

semoga membantu
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