a, Ta có : \(P_{\left(x\right)}=x^2+5x^4-3x^3+x^2+4x^4+3x^2-x+5\)
\(=9x^4-3x^3+5x^2-x+5\)
Ta có : \(Q_{\left(x\right)}=x-5x^3-x^2-x^4+4x^3-x^2+3x-1\)
\(Q_{\left(x\right)}=-x^4-x^3-2x^2+4x-1\)
b, Ta có : \(P_{\left(x\right)}+Q_{\left(x\right)}=9x^4-3x^3+5x^2-x+5-x^4-x^3-2x^2+4x-1\)
\(=8x^4-4x^3+3x^2+3x+4\)
Ta có : \(P_{\left(x\right)}-Q_{\left(x\right)}=9x^4-3x^3+5x^2-x+5+x^4+x^3+2x^2-4x+1\)
\(=10x^4-2x^3+7x^2-5x+6\)