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U₃ = a + 2b = 15
U₁₀ = a + 9b = 43
Ditanya: Jumlah 25 Suku Pertama (S₂₅)?
Dijawab:
b = (U₁₀ - U₃) / (10 - 3)
b = (43 - 15) / 7
b = 28 / 7
b = 4
a + 2b = 15
a + 2(4) = 15
a = 15 - 8
a = 7
Sn = n / 2. (2a + (n - 1)b)
S₂₅ = 25 / 2. (2(7) + (25 - 1)4)
S₂₅ = 25 / 2. (14 + 96)
S₂₅ = 25 / 2. (110)
S₂₅ = 1.375