A = (2X - 3)2 - (X + (X + 5) + 2 = 4X2 - 12X + 9 - X2 - 6X - 5 + 2 = 3X2 - 18X + 6 = 3(X2 - 6X + 2) = 3[(X - 3)2 - 7] ≥ 3

1) A = (2x - 3)

²

- (x + 1)(x + 5) + 2 = 4x

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- 12x + 9 - x

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- 6x - 5 + 2 = 3x

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- 18x + 6 = 3(x

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- 6x + 2) = 3[(x - 3)

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- 7] ≥ 3.(-7) = -21Dấu "=" xảy ra khi x - 3 = 0 ⇔ x = 3. Vậy MinA = -21 ⇔ x = 3