Đáp án+Giải thích các bước giải:

 Bài `13:`

`x^{2}+7xy+12y^{2}-20y-5x`

`=(x^{2}+4xy+3xy+12y^{2})-(5x+20y)`

`=[x.(x+4y)+3y.(x+4y)]-5.(x+4y)`

`=(x+3y).(x+4y)-5.(x+4y)`

`=(x+4y).(x+3y-5)`

Bài `14:`

`x.(y^{2}-z^{2})+y.(z^{2}-x^{2})+z.(x^{2}-y^{2})`

`=xy^{2}-xz^{2}+yz^{2}-x^{2}y+x^{2}z-y^{2}z`

`=(xy^{2}-y^{2}z)+(x^{2}z-xz^{2})-(x^{2}y-yz^{2})`

`=y^{2}.(x-z)+xz.(x-z)-y.(x^{2}-z^{2})`

`=(x-z).(y^{2}+xz)-y.(x-z).(x+z)`

`=(x-z).[y^{2}+xz-y.(x+z)]`

`=(x-z).(y^{2}+xz-xy-yz)`

`=(x-z).[y.(y-z)-x.(y-z)]`

`=(x-z).(y-x).(y-z)`