a, M(x)+N(x)=(3x3 -3x+x2) + ( 2x2-x+3x3+9)
=(3x3+3x3) +(-3x-x)+(x2+2x2)+9
= 6x3 -4x +3x2+9
b, M(x)+N(x)-P(x)= 6x3+3x2+2x
6x3 -4x +3x2+9 - P(x) =6x3+3x2+2x
6x3 -4x +3x2+9 -(6x3+3x2+2x) =P(x)
(6x3-6x3) +(3x2-3x2) +(-4x-2x)+9=P(x)
-6x+9 = P(x)
c, Cho P(x)= -6x+9=0
=> -6x+9 =0
-6x=-9
x =9/6
=> x = 3/2
Chúc bạn học tốt
a) \(M\left(x\right)+N\left(x\right)=3x^3-3x+x^2+2x^2-x+3x^3+9\)
\(M\left(x\right)+N\left(x\right)=\left(3x^3+3x^3\right)+\left(x^2+2x^2\right)+\left(-3x-x\right)+9\)
\(M\left(x\right)+N\left(x\right)=6x^3+3x^2-4x+9\)
b) \(M\left(x\right)+N\left(x\right)-\left(6x^3+3x^2+2x\right)=P\left(x\right)\)
\(6x^3+3x^2-4x+9-6x^3-3x^2-2x=P\left(x\right)\)
\(P\left(x\right)=-6x+9\) c) Ta có \(P\left(x\right)=0\Leftrightarrow-6x+9=0\) \(\Leftrightarrow-6x=0-9=-9\) \(\Leftrightarrow x=\dfrac{-9}{-6}=\dfrac{3}{2}\) Vậy \(x=\dfrac{3}{2}\) là nghiệm của đa thúc P(x)