Đáp án `+` Giải thích các bước giải:

`3)` `(x-2)^3 -x(x-2)(x+2)+6x(x-3)`

`=x^3 -6x^2 +12x-8-x(x^2 -4)+6x^2 -18x`

`=x^3 -6x^2 +12x-8-x^3 +4x+6x^2 -18x`

`=(x^3 -x^3)+(-6x^2 +6x^2)+(12x+4x-18x)-8`

`=-2x-8`

`4)` `x(x-5)(x+5)-(x-5)^3 +100x`

`=x(x^2 -25)-(x^3 -15x^2 +75x-125)+100x`

`=x^3 -25x-x^3 +15x^2 -75x+125+100x`

`=(x^3 -x^3)+15x^2 -(25x+75x)+100x+125`

`=15x^2 -100x+100x+125`

`=15x^2 +125`