Đáp án+Giải thích các bước giải:

Bài `11:`

`x^2 +6xy+5y^2 −5y−x`

`=x^2 +xy+5xy+5y^2 −5y−x `

`=x(x+y)+5y(x+y)−(x+5y)`

`=(x+y)(x+5y)−(x+5y) `

`=(x+5y)(x+y−1) `

Bài `12:`

`x^4 +2019x^2 +2018x+2019`

`=x^4 +2019x^2 +2019x−x+2019 `

`=x^4 −x+2019x^2 +2019x+2019 `

`=x(x^3 −1)+2019(x^2 +x+1) `

`=x(x−1)(x^2 +x+1)+2019(x^2 +x+1) `

`=(x^2 +x+1)[x(x−1)+2019] `

`=(x^2 +x+1)(x^2 −x+2019) `