phân tích đa thức thành nhân tử x^2-49x x^5-4x^4+4x^3 2xy-x^2-y^2+81 5x5y+40x^2y giúp mình nha

`a)x^2-49x.`

`=x.(x-49).`

`b)x^5-4x^4+4x^3.`

`=x^3.(x^2-4x+4).`

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

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

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

`c)2xy-x^2-y^2+81.`

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

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

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

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

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

`=[9+(x-y)].[9-(x-y)].`

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

`d)5x^5y+40x^2y.`

`=5x^2y.(x^3+8).`