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Sn : n/2 (2a + (n-1) b
atau
Sn = n/2(a + Un)
Sn = n/2 (a+2n-5)
kita cari nilai a nya.
a = U1. Iya to?
U1 = 3 (1) - 5
U1 = 3 - 5
U1 = -2
JADI
Sn = n/2 (-2 + 2n - 5)
Sn = n/2 (2n - 7)
atau juga bisa ditulis
Sn = n (2n-7) /2