Giải:
`a, P(x) + Q(x)`
`= (x^3 - 6x + 2) + (2x^2 - 4x^3 + x - 5)`
`= x^3 - 6x + 2 + 2x^2 - 4x^3 + x - 5`
`= (x^3 - 4x^3) + 2x^2 - (6x - x) + (2 - 5)`
`= -3x^3 + 2x^2 - 5x - 3`
`b, P(x) - Q(x)`
`= (x^3 - 6x + 2) - (2x^2 - 4x^3 + x - 5)`
`= x^3 - 6x + 2 - 2x^2 + 4x^3 - x + 5`
`= (x^3 + 4x^3) - 2x^2 - (6x + x) + (2 + 5)`
`= 5x^3 - 2x^2 - 7x + 7`