a)
P(x) = -2x³ + 5x + 5x³ - 2x + 3x² - 3
= (-2x³ +5x³)+3x²+(5x-2x)-3
= 3x³+3x²+3x-3
Q(x) = 3x³ - x² + 2x - 5x + 4x² + 5
= 3x³ +(-x²+4x²)+(2x-5x)+5
= 3x³+3x²-3x+5
b)Ta có : A(x)=P(x)+Q(x)
A(x)=(3x³+3x²+3x-3)+(3x³+3x²-3x+5)
= 3x³+3x²+3x-3+3x³+3x²-3x+5
= (3x³+3x³)+(3x²+3x²)+(3x-3x)+(-3+5)
= 6x³ + 6x² +2
Ta có : B(x) = P(x) - Q(x)
B(x)=(3x³+3x²+3x-3)-(3x³+3x²-3x+5)
= 3x³+3x²+3x-3-3x³-3x²+3x-5
= (3x³-3x³)+(3x²-3x²)+(3x+3x)+(-3-5)
= 6x-8