Đáp án+Giải thích các bước giải:

 `27)(-x-2).(x+1)-(x-5).(-x+1)`

`=(-x^{2}-x-2x-2)-(-x^{2}+x+5x-5)`

`=-x^{2}-3x-2+x^{2}-6x+5`

`=-9x+3`

`28)5x.(x-3).(x-1)-4x.(x^{2}-2x)`

`=5x.(x^{2}-x-3x+3)-4x^{3}+8x^{2}`

`=5x.(x^{2}-2x+3)-4x^{3}+8x^{2}`

`=5x^{3}-10x^{2}+15x-4x^{3}+8x^{2}`

`=x^{3}-2x^{2}+15x`

`29)-4x.(x+3).(x-4)-3x.(x^{2}-x+1)`

`=-4x.(x^{2}-4x+3x-12)-3x^{3}+3x^{2}-3x`

`=-4x.(x^{2}-x-12)-3x^{3}+3x^{2}-3x`

`=-4x^{3}+4x^{2}+48x-3x^{3}+3x^{2}-3x`

`=-7x^{3}+7x^{2}+45x`

`30)-3.(x+4).(x-7)+7.(x-5).(x-1)`

`=-3.(x^{2}-7x+4x-28)+7.(x^{2}-x-5x+5)`

`=-3.(x^{2}-3x-28)+7.(x^{2}-6x+5)`

`=-3x^{2}+9x+84+7x^{2}-42x+35`

`=4x^{2}-33x+119`

`31)4x.(x^{2}-x+3)-(x-6).(x-5)`

`=4x^{3}-4x^{2}+12x-(x^{2}-5x-6x+30)`

`=4x^{3}-4x^{2}+12x-x^{2}+11x-30`

`=4x^{3}-5x^{2}+23x-30`