$\\$
`a,`
`A=(x+y)^3 +x^3`
`->A=(x+y+x)[(x+y)^2 - (x+y)x +x^2]`
`->A = (2x+y) [(x+y)^2 - (x+y)x +x^2]`
`->A=0 . [(x+y)^2 - (x+y)x +x^2]`
`->A=0`
Vậy `A=0` khi `2x+y=0`
$\\$
`b,`
`B=x^3 -y^3 - 3xy`
`->A = (x-y)(x^2 +xy+y^2) - 3xy`
`->A=1 . (x^2 +xy+y^2)-3xy`
`->A=x^2 +xy+y^2 - 3xy`
`->A=x^2 -2xy+y^2`
`->A=(x-y)^2`
`->A=1^2`
`->A=1`
Vậy `A=1` khi `x-y=1`
