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U5 = 11
U8 + U12 = 52
Ditanya:
S8 = ?
Jawab:
U5 = 11
a+4b = 11 ......................... (1)
U8 + U12 = 52
a+7b + a+11b = 52
2a+18b = 52 .................... (2)
Subtitusikan 1 dan 2:
2a + 18b = 52 ....... (dikali 1)
a + 4b = 11 ....... (dikali 2)
____________
2a + 18b = 52
2a + 8b = 22
____________ -
10b = 30
b = 3
Masukkan nilai b ke 1:
a + 4b = 11
a + 4(3) = 11
a + 12 = 11
a = -1
Jadi,
S8 = n/2 (2a + (n-1)b)
S8 = 8/2 (2(-1) + (8-1)3)
S8 = 4(-2 + 21)
S8 = 4(19)
S8 = 76