(x2 + x)2 + 4x2 + 4x - 12 (sửa đề)
= (x2 + x)2 + 4(x2 + x) - 12
Đặt x2 + x = y, ta có:
y2 + 4y - 12
= y2 + 2y - 6y - 12
= y(y + 2) - 6(y + 2)
= (y - 6)(y + 2)
= (x2 + x - 6)(x2 + x + 2)
= (x2 - 2x + 3x - 6)(x2 + x + 2)
= (x + 3)(x - 2)(x2 + x + 2)