MN giải giúp bé bài này với ạ

a) `(x+y)^{3}+(x-y)^{3}-6x^{2}y`

`=x^{3}+3x^{2}y+3xy^{2}+y^{3}+x^{2}-3x^{2}y+3xy^{2}-y^{3}-6x^{2}y`

`=2x^{3}+6xy^{2}-6x^{2}y``

b) `(x+2)^{3}+(x-2)^{3}-2x(x^{2}+12)`

`=(x+2+x-2)^{3}-3(x+2)(x-2)(x+2+x-2)-2x(x^{2}+12)`

`=(2x)^{2)-6x(x^{2}-2^{2})-(2x^{3}+24x)`

`=8x^{3}-6x^{3}+24x-2x^{3}-24x` 

`=0`

c)`(x-3)^{3}-(x-3)(x^{2}+3x+9)+6(x+1)^{2}` `=x^{3}-9x^{2}+27X-27-(x^{3}-27)+6(x^{2}+2x+1)` `=x^{3}-9x^{2}+27x-27-x^{3}+27+6x^{2}+12x+6` `=-3x^{2}+39x+6`

d) `(x^{4}-3x^{2}+9)(x^{2}+3)-(3+x^{2})^{3}`

`=(x^{2})^{3}+3^{3}-(x^{2}+3)^{3}`

`=x^{6}-3x^{2}+3x-1-x^{3}-3x^{2}-3x-1+6x^{2}-6`

`=-8`