`(x^2 - 5x + 1)^2 + (5x - 1)^2 + (5x - 1)(x^2 - 5x + 1)`
`= (x^2 - 5x + 1)^2 + 2(5x - 1)(x^2 - 5x + 1) + (5x - 1)^2 - (5x - 1)(x^2 - 5x + 1)`
Áp dụng đẳng thức `x^2 + 2xy + y^2 = (x + y)^2`, biểu thức trở thành:
`(x^2 - 5x + 1 + 5x - 1)^2 - (5x - 1)(x^2 - 5x + 1)`
`= (x^2)^2 - (5x. x^2 - 5x. 5x + 5x - x^2 + 5x - 1)`
`= x^4 - (5x^3 - 25x^2 + 5x - x^2 + 5x - 1)`
`= x^4 - (5x^3 - 26x^2 + 10x - 1)`
`= x^4 - 5x^3 + 26x^2 - 10x + 1`
