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方程 x^2 + y^2 + 4x - 2y + 10 = 0 表示圆吗？（答：不——半径 ^2 < 0 。）
方程 x^2 + y^2 + 4x - 2y + 10 = 0 表示圆吗？（答：不——半径 ^2 < 0 。）
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