P ( x ) + Q(x) = 2x^5 - 3x^4 + 5x+ 2x^2 - 7 + 7x - 4x^2 + 5x^4 - 3x^5 + 2x^3 - 9
P ( x ) + Q(x) = ( 2x^5 - 3x^5 ) + ( 5x^4 - 3x^4 ) + 2x^3 +( 2x^2 - 4x^2 ) + ( 5x + 7x ) - ( 7 + 9 )
P ( x ) + Q(x) = -x^5 + 2x^4 + 2x^3 - 2x^2 + 12x - 16
Vậy P ( x ) + Q(x) = -x^5 + 2x^4 + 2x^3 - 2x^2 + 12x - 16