Đáp án:
`(x^2 + 9)^2 + 8x(x^2 + 9) + 12x^2`
`= (x^2 + 9)^2 + 8x(x^2 + 9) + 16x^2 - 4x^2`
`= [(x^2 + 9)^2 + 2.(x^2 + 9).4x + (4x)^2] - 4x^2`
`= (x^2 + 9 + 4x)^2 - (2x)^2`
`= (x^2 + 9 + 4x + 2x)(x^2 + 9 + 4x - 2x)`
`= (x^2 + 6x + 9)(x^2 + 2x + 9)`
`= (x^2 + 2.x.3 + 3^2)(x^2 + 2x + 9)`
`= (x + 3)^2 (x^2 + 2x + 9)`
* Áp dụng hằng đẳng thức:
`(A + B)^2 = A^2 + 2AB + B^2`
`A^2 - B^2 = (A - B)(A + B)`