Đáp án + Giải thích các bước giải:

Bài `4A:`

`a)` `x.(x-y)+y.(x-y)`

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

`=` `x^2-y^2`

`b)` `x^2.(3x-2y+y^2)+3y.(x^2+4x+5)-12xy.(xy+1)`

`=` `3x^3 − 2x^2y + x^2y^2+3x^2y + 12xy + 15y− 12x^2y^2 − 12xy`

`=` `3x^3 + (−2 + 3)x^2y + (1 − 12)x^2y^2 + (12 − 12)xy + 15y`

`=` `3x^3 + x^2y − 11x^2y^2 + 15y`

`c)` `x^3.(2x + 3y) − 4y.(x^3 + 3x) + 12xy`

`=` `2x^4 + 3x^3y − 4x^3y − 12xy + 12xy`

`=` `2x^4 − x^3y`

`=` `x^3(2x − y)`

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