Cách làm cho bạn:
Ta có:
a) $A(x)+B(x)+C(x)=A(x)+(B(x)+C(x))$ $=2x^{3}−2x^{2}+x−4+3x^{3}−2x+3−x3+1$
$=2x^{3}−2x^{2}+x−4+2x^{3}−2x+4$
=$4x^{3}−2x^{2}−x$
b) A(x)−B(x)−C(x)
$=A(x)−(B(x)+C(x))$
$=2x^{3}−2x^{2}+x−4−(3x^{3}−2x+3−x3+1)$
$=2x^{3}−2x^{2}+x−4−2x^{3}+2x−4$
$=−2x^{2}+3x−8$
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