9*(а₁ + 10d) = а₁ + 18d
8а₁ = -72d
а₁ = -9d
а₁₉ = -9d + 18d = 9d
S₁₉ = (а₁ + а₁₉)*19/2 = (-9d + 9d)*19/2 = 0
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Verified answer
9*(а₁ + 10d) = а₁ + 18d
8а₁ = -72d
а₁ = -9d
а₁₉ = -9d + 18d = 9d
S₁₉ = (а₁ + а₁₉)*19/2 = (-9d + 9d)*19/2 = 0