a, \(P\left(x\right)=-x^4+3x^3-6x^2+2x+\dfrac{1}{2}\)
\(Q\left(x\right)=5x^5-x^3+x^2-7x-\dfrac{1}{4}\)
b, Ta có \(P\left(x\right)+Q\left(x\right)=5x^5-x^4+2x^3-5x^2-5x+\dfrac{1}{4}\)
\(P\left(x\right)-Q\left(x\right)=-x^4+4x^3-7x^2+9x+\dfrac{3}{4}-5x^5\)