Bài `6`

`(x^2-4)(x+2)`

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

`=x^3+2x^2-4x-8`

Bài `7`

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

`=(x-1+x+1)[(x-1)^2-(x-1)(x+1)+(x+1)^2]`

`=2x[x^2-2x+1-(x^2-1)+x^2+2x+1]`

`=2x(x^2-2x+1-x^2+1+x^2+2x+1)`

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

`=2x^3+6x`

Bài `8`

`(x+2)^2-2(x^2+4x+4)`

`=x^2+4x+4-2x^2-8x-8`

`=(x^2-2x^2)+(4x-8x)+(4-8)`

`=-x^2-4x-4`

Bài `9`

`(x-2)(x+2)(x^2+4)`

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

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

`=x^4-16`

Bài `10`

`(x+y)^2-(x-y)^2`

`=[(x+y)-(x-y)][(x+y)+(x-y)]`

`=(x+y-x+y)(x+y+x-y)`

`=2y·2x`

`=4xy`