Đặt: `{(2a+b-c=x), (2b+c-a=y), (2c+a-b=z):}`
`=> x + y + z = 2a + b- c + 2b + c - a + 2c + a - b`
`<=> x + y + z = 2a + 2b + 2c = 2(a+b+c)`.
`-> (x+y+z)^3 = 8(a+b+c)^3`
PT trở thành:
`(x+y+z)^3 - x^3 - y^3 - z^3`.
`= 3(x+y)(y+z)(z+x)`
`= 3(2a+b-c+2b+c-a)(2b+c-a+2c+a-b)(2c+a-b+2a+b-c)`
`= 3(a+3b)(b+3c)(c+3a)`.