y = ax² + bx + c= a(x² + bx/a) + c= a[x² + bx/a + (b/2a)²] - a*(b/2a)² + c= a(x + b/2a)² - b²/4a + c= a(x + b/2a)² + (4ac - b²)/4a顶点是[- b/2a,(4ac - b
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