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Sn = 1 180 000
a = 40 000
b = 2 000
Sn = (n/2)(2a+(n-1)b)
1180 = (n/2)(78 + 2n)
1180 = n² + 39n
0 = n² + 39n - 1180
0 = (n-20)(n+59)
n = 20 atau n = -59 ← ambil positif aja
n = 20 bulan............opsi D