Cách làm cho bạn:
Ta có:
$P(x)+Q(x)+R(x)= 3x^{4}−2x^{2}+8x-10+4x^{3}−6x^{2}+7x−1-3x^{4}+5x^{2}−8x−5$
=$3x^{2}−15+4x^{3}−6x^{2}+7x−1$
$=4x^{3}−3x^{2}+7x−16$
$P(x)−Q(x)−R(x)=3x^{4}−2x^{2}+8x-10-(4x^{3}−6x^{2}+7x−1)-(−3x^{4}+5x^{2}−8x−5)$
$=6x^{4}−4x^{3}−x^{2}+9x−4$
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