a) P(x) = 6x^3 - 4x + 8 - x
P(x)=6x^3+(4-1)x+8
P(x)=6x^3+3x+8
Q(x) =-6x^3 + 3x - 3 + 2x - x^2 - 2
Q(x)=-6x^3+(3+2)x+(-3-2)-x^2
Q(x)=-6x^3+5x+(-5)-x^2
b)M(x)=P(x)+Q(x)
M(x)=(6x^3+3x+8)+ (-6x^3+5x+(-5)-x^2]
M(x)=6x^3+3x+8+(-6)x^3+(-5)-x^2
M(x)=(6-6)x^3+(8+5)+3x-x^2
M(x)=0+13+3x-x^2
M(x)=-x^2+3x+13
vậy đa thức M(x)= -x^2+3x+13
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