`a) Q(x) = 4x - 7x^3 - 5x^2 - 3 + 2x^3`
` = (-7x^3 + 2x^3) - 5x^2 + 4x-3`
` = -5x^3 - 5x^2 + 4x-3`
`P(x) = -6x^3 - 5x + 9x^2 + 7-4x`
` = -6x^3 + 9x^2 - (5x+4x) +7`
` = -6x^3 + 9x^2 - 9x + 7`
`b)` Ta có:
`Q(x) + P(x) = (-5x^3 - 5x^2 + 4x-3) + (-6x^3 + 9x^2 - 9x+7)`
` = -5x^3 - 5x^2 + 4x-3 - 6x^3 + 9x^2 - 9x + 7`
` = -(5x^3 + 6x^3) +(-5x^2 + 9x^2) + (4x-9x) - (3-7)`
` = -11x^3 + 4x^2 - 5x + 4`
`Q(x) - P(x) = (-5x^3 - 5x^2 + 4x-3) - (-6x^3 + 9x^2 - 9x+7)`
` = -5x^3 - 5x^2 + 4x - 3 + 6x^3 - 9x^2 + 9x - 7`
` = (-5x^3 + 6x^3) + (-5x^2 - 9x^2) + (4x+9x) - (3+7)`
` = x^3 - 14x^2 + 13x - 10`