a) Ta có :P(x)+Q(x) = (6x3+5x-3x2-1) + (5x2-4x3-2x+7)
= 6x3+5x-3x2-1 + 5x2-4x3-2x+7
=6x3-4x3-3x2+5x2+5x-2x-1+7
=2x3+2x2+3x+6
b) ta có : P(x)-Q(x) = (6x3+5x-3x2-1) - (5x2-4x3-2x+7)
=6x3+5x-3x2-1 - 5x2+4x3+2x-7
=6x3+4x3-3x2- 5x2+5x+2x-1-7
=10x3-8x2+7x-8