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S₃ = a₁ + a₂ + a₃
9 = a₁ + a₂
15 = a₁ + a₂ + a₃
9 = a₁ + a₂
15 = 9 + a₃
a₃ = 6
9 = a₁ + a₂
a₁ + a₂ = a₁ + a₁ + r = 2a₁ + r
a₃ = a₁ + 2r
9 = 2a₁ + r
6 = a₁ + 2r /*(-2)
9 = 2a₁ + r
-12 = -2a₁ - 4r
-3 = -3r
9 = 2a₁ + r
r=1
9 = 2a₁ + 1
r = 1
a₁ = 4
an = 4 + (n - 1)*1 = 4 + n - 1 = 3 + n