\(c\text{â}ua:x^3-4x=x\left(x^2-4\right)=x\left(x^2-2^2\right)=x\left(x-2\right)\left(x+2\right)\)

\(c\text{â}ub:x^2-2xy-9+y^2=\left(x^2-2xy+y^2\right)-9=\left(x-y\right)^2-3^2=\left(x-y+3\right)\left(x+y-3\right)\)

đề có bị sai hay thiếu ko