Q(x) = 4x – $7x^{3}$- $5^{2}$ -3 + $2x^{3}$ P ( X) = $-6x^{3}$- 5x + $9x^{2}$ + 7 – 4x a . ) Thu gọn và sắp xếp theo thứ tự giảm dần của biến b .)

`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`