phan tích các đa thức sau thành nhân tử: a) x^2-xy+3x-3y ; b) x^3-4x^2-xy^2+4x; c) (x+1).(x+2).(x+3).(x+4)-3 mn làm ơn giúp mik ạ vui lòng trình bà

`a)x^2-xy+3x-3y`

`=(x^2-xy)+(3x-3y)`

`=x(x-y)+3(x-y)`

`=(x-y)(x+3)`

`b)x^3-4x^2-xy^2+4x`

`=x(x^2-4x-y^2+4)`

`=x[(x^2-4x+4)-y^2]`

`=x[(x-2)^2-y^2]`

`=x(x-2+y)(x-2-y)`

`c)(x+1)(x+2)(x+3)(x+4)-3`

`=[(x+1)(x+4)].[(x+2)(x+3)]-3`

`=(x^2+5x+4)(x^2+5x+6)-3`

Đặt `x^2+5x+4=t`

`t.(t+2)-3`

`=t^2+2t-3`

`=t^2+3t-t-3`

`=(t^2+3t)-(t+3)`

`=t(t+3)-(t+3)`

`=(t+3)(t-1)`

`=(x^2+5x+4+3)(x^2+5x+4-1)`

`=(x^2+5x+7)(x^2+5x+3)`

Vậy `(x+1)(x+2)(x+3)(x+4)-3=(x^2+5x+7)(x^2+5x+3)`