对于二次方程 ax² + bx + c = 0,其求根公式是?
x = [-b ± √(b² + 4ac)] / (2a)
x = [-b ± √(b² - 4ac)] / (2a)
x = [b ± √(b² - 4ac)] / (2a)
x = [b ± √(b² + 4ac)] / (2a)
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