Đáp án:
`x^3+y^3+z^3-3xyz`
`= (x + y)³ - 3xy(x + y) + z³ - 3xyz `
`= ( x + y)^3 - 3xy( x + y) - 3xyz + z^3`
`= (x + y)³ + z³ - 3xy(x + y + z) `
`= (x + y + z)³ - 3(x + y + z)(x + y)z - 3xy(x + y + z) `
`= (x + y + z)³ - 3(x + y + z)(xy + yz + zx)`
`= ( x + y+ z)[ ( x + y + z)^2 - 3(xy + yz + zx)]`
`= (x + y + z)[(x + y + z)² - 3xy - 3yz - 3zx)] `
`= (x + y + z)(x² + y² + z² - xy - yz - zx)`
