phân tích đa thức thành nhân tử help me Hmu:((

`k)x^2-6xy+9y^2-25z^2`

`=(x^2-6xy+9y^2)-25z^2`

`=[x^2-2.x.3y+(3y)^2]-(5z)^2`

`=(x-3y)^2-(5x)^2`

`=(x-3y-5z)(x-3y+5x)`

`l)81-x^2+4xy-4y^2`

`=81-(x^2-4xy+4y^2)`

`=81-[x^2-2.x.2y+(2y)^2]`

`=9^2-(x-2y)^2`

`=(9-x+2y)(9+x-2y)`

`m)x^2+6x-y^2+9`

`=(x^2+6x+9)-y^2`

`=(x^2+2.x.3+3^2)-y^2`

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

`=(x+3-y)(x+3+y)`

`=(x-y+3)(x+y+3)`