a) `Q(x)= 2A(x) + 3B(x)`
`=> Q(x)= 2(1/2 x^3 - 2x^2 + x-1) + 3(x^3 - 2x^2 +x -5)`
`= x^3 - 4x^2 + 2x -2 + 3x^3 - 6x^2 + 3x -15`
`= (x^3 + 3x^3) - (4x^2 + 6x^2) - (2+15) + (2x+3x)`
`= 4x^3 - 10x^2 - 17 + 5x`
b) `P(x) =A(x) -B(x)`
`=> P(x)=( 1/2 x^3 - 2x^2 + x-1 )- (x^3-2x^2 + x-5)`
`= 1/2 x^3 - 2x^2 + x-1 -x^3 + 2x^2 - x +5`
`= (1/2 x^3 - x^3) - (2x^2 - 2x^2) + (x-x) - (1-5)`
`= -1/2x^3 + 4`
Xét `P(x) =0`
`=> -1/2 x^3 + 4 =0`
`=> -1/2 x^3 = -4`
`=> 1/2 x^3 = 4`
`=> x^3 = 4 : 1/2`
`=> x^3 = 4. 2`
`=> x^3 =8`
`=> x^3 = 2^3`
`=> x= 2`
Vậy nghiệm của `P(x)` là `2`
