Tentukan n jika:
a. -3, 4, 11...........+ n =285
a. -3, 4, 11...........+ n =285
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b = 7
Sn = n/2 (2(-3) + (n-1)7)
285 = n/2 (-6 + 7n - 7)
570 = n (7n - 13)
7n^2 - 13 n - 570 = 0
(7n+57)(n - 10) = 0
n = -57/7 atau n = 10
jadi n = 10