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b = 9-7 = 2
Sn = n/2 (2a + (n-1) b)
432 = n/2 (2(7) + 2n -2)
864 = n ( 14 + 2n -2)
864 = 12 n + 2 n²
432 = 6n + n²
n² + 6n - 432 = 0
(n+24)(n-18) = 0
n = -24 atau n = 18
Jadi banyak suku 18 (ambil positifnya)
U18 = a + 17b
= 7 + 17 ( 2)
= 7 + 34
= 41
Suku terakhiir = 41
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