Tổng h(x) + g(x)
$= ( 9 - x^5 + 4x - 2x^3 + x^2 - 7x^4 ) + ( x^5 - 9 + 2x^2 + 7x^4 +2x^3 - 3x)$
$= 9 - x^5 + 4x - 2x^3 + x^2 - 7x^4 + x^5 - 9 + 2x^2 + 7x^4 +2x^3 - 3x$
$= (-x^5 + x^5) - (7x^4 - 7x^4) - (2x^3 -2x^3) + (x^2 +2x^2 ) (4x - 3x) + (9-9)$
$= 0 - 0 - 0 +3x^2+ x + 0$
$= 3x^2 +x $