`@Ryley`

`5x^2+5y^2-x^2z+2xyz-y^z-10xy`

`= (5x^2+5y^2)-(x^2z+y^2z)+(2xyz-10xy)`

`= 5(x^2+y^2)-z(x^2+y^2)+2xy(z-5)`

`= (5-z).(x^2+y^2)-2xy(5-z)`

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

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

`= (5-z)(x-y)^2`
Áp dụng: 
`***HĐT:(a-b)^{2}=a^{2}-2ab+b^{2}`