Tentukan rumus jumlah n suku pertama deret aritmatika jika u7=15&s7=168

S7 = 7/2 (a + U7)
168 = 7/2 (a + 15)
336 / 7 = 15 + a
48 = 15 + a
a = 33

U7 = a + 6b
15 = 33 + 6b
-18 = 6b
b = -3

Sn = n/2 (2a + (n-1)b)
Sn = n/2 (66 + (n-1)(-3))
Sn = n/2 (69 - 3n)

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