a.(2x-y) (4x^2+2xy+y^2) b.(x+3) (x^2-3x+9)

`\color{red}{@tnt}`

`a) (2x-y)(4x^2 + 2xy +y^2)`

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

`= 8x^3 + (-4x^2 y +4x^2 y) + (-2xy^2 +2xy^2) -y^3`

`= 8x^3 -y^3`

`---`

`b) (x+3)(x^2 -3x +9)`

`= x^3 +3x^2 -3x^2 -9x+9x +27`

`= x^3 +(3x^2 -3x^2) + (-9x+9x) +27`

`= x^3 +27`

`@` Áp dụng `HĐT` :

`A^3 -B^3 = (A-B)(A^2 +AB +B^2)`

`A^3 +B^3 = (A+B)(A^2 -AB +B^2)`

~~Hok tốt~~