Tham khảo:

Bài ` 3: `

` 1) (x - 2)(-3 + x) `

` = x (-3 + x) - 2 (-3 + x) `

` = (-3x + x^2) + (6 - 2x) `

` = x^2 - 3x - 2x + 6 `

` = x^2 - 5x + 6 `

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` 2) (x - 3)(2x + 5) `

` = x (2x + 5) - 3( 2x + 5) `

` = (2x^2 + 5x) + (-6x - 15) `

` = 2x^2 + 5x - 6x - 15 `

` = 2x^2 - x - 15 `

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` 4) (x - 3)(5x - 4) `

` = x (5x - 4) - 3( 5x - 4) `

` = (5x^2 - 4x) + (-15x + 12) `

` = 5x^2 - 4x - 15x + 12 `

` = 5x^2 - 19 + 12 `

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` 5) (x - 3)(-2x - 4) `

` = x (-2x - 4) - 3( -2x - 4) `

` = (-2x^2 - 4x) + (6x + 12) `

` = 2x^2 - 4x + 6x + 12 `

` = -2x^2 + 2x + 12 `

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` 7) (x + 3)(x^2 + 3x - 5) `

` = x (x^2 + 3x - 5) + 3 (x^2 + 3x - 5) `

` = (x^3 + 3x^2 - 5x) + (3x^2 + 9x - 15) `

` = x^3 + 3x^2 + 3x^2 - 5x + 9x - 15 `

` = x^3 + 6x^2 + 4x - 15 `

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` 8) (x - 5)(x^2 - 2x + 3) `

` = x( x^2 - 2x + 3) - 5(x^2 - 2x + 3) `

` = (x^2 - 2x^2 + 3x) + (-5x^2 + 10x - 15) `

` = x^3 - 2x^2 - 5x^2 + 3x + 10x - 15 `

` = x^3 - 7x^2 + 13x + 15 `