Y=2分之1x的平方+x-2分之5 = 1/2*(x + 2x) - 5/2 = 1/2*(x + 1) - 3 顶点（-1,-3）对称轴: x = -1 令1/2*(x + 1) - 3 = 0,得：x = - 1 ±√6 所以,AB = 丨X1 - X2丨 = 2√6
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