`#Sad`
`4)`
`\text{→Chứng minh:}`
`(xy+1)(x^2y^2-xy+1)+(x^3-1)(1-y^3) = x^3+y^3`
`\text{Ta có:}`
`\text{VT}` `= (xy+1)(x^2y^2-xy+1)+(x^3-1)(1-y^3)`
`= x^3y^3-x^2y^2+xy+x^2y^2-xy+1+x^3-x^3y^3-1+y^3`
`= (x^3y^3-x^3y^3)+(-x^2y^2+x^2y^2)+(xy-xy)+(1-1)+x^3+y^3`
`= x^3+y^3` `=` `\text{VP}` `AA` `x; y` `\text{(→đpcm)}`