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S₁ = 3×1² + 5×1 = 3 + 5 = 8
a₁ = S₁ = 8
S₂ = 3×2² + 5×2 = 12 + 10 = 22
S₂ = a₁ + a₂
a₁ + a₂ = 22
8 + a₂ = 22
a₂ = 22 - 8
a₂ = 14
r = a₂ - a₁ = 14 - 8 = 6
an = a₁ + (n - 1)r
a₁₀₃ = 8 + (103 - 1) * 6
a₁₀₃ = 8 + 102 * 6
a₁₀₃ = 8 + 612
a₁₀₃ = 620