Hướng dẫn trả lời:
Bài 3:
1) `6x^2 - 2x.(3x - 5).`
`= 6x^2 - 2x.3x - (-2x).5.`
`= 6x^2 - 6x^2 + 10x.`
`= (6x^2 - 6x^2) + 10x.`
`= 10x.`
2) `2x - x^2 + x.(2x + 1).`
`= 2x - x^2 + x.2x + x.1.`
`= 2x - x^2 + 2x^2 + x.`
`= (- x^2 + 2x^2) + (2x + x).`
`= x^2 + 3x.`
3) `3x - 5x.(x - 4) - 5x^2.`
`= 3x - 5x.x - (-5x).4 - 5x^2.`
`= 3x - 5x^2 + 20x - 5x^2.`
`= (- 5x^2 - 5x^2) + (3x + 20x).`
`= - 10x^2 + 23x.`
4) `x^2.(x - y) + xy.(1 + xy).`
`= x^2.x - x^2.y + xy.1 + xy.xy.`
`= x^3 - x^2y + xy + (xy)^2.`
`= x^3 - x^2y + xy + x^2y^2.`
