*Lời giải :
`a)`
\(\left\{ \begin{array}{l}P(x)=x^2+5x^4-3x^3+x^2-5x^4+3x^3-x+5\\Q(x)=x-5x^3-x^2+5x^3-x^2+3x-1\end{array} \right.\)
`=` \(\left\{ \begin{array}{l}P(x)=(x^2+x^2)+(5x^4-5x^4)+(-3x^3+3x^3)-x+5\\Q(x)=(x+3x)+(-5x^3+5x^3)+(-x^2-x^2)-1\end{array} \right.\)
`=` \(\left\{ \begin{array}{l}P(x)=2x^2-x+5\\Q(x)=4x-2x^2-1\end{array} \right.\)
Sắp xếp \(\left\{ \begin{array}{l}P(x)\\Q(x)\end{array} \right.\) theo lũy thừa giảm dần của biến
\(\left\{ \begin{array}{l}P(x)=2x^2-x+5\\Q(x)=-2x^2+4x-1\end{array} \right.\)
`b)`
`P(x) + Q(x)=(2x^2-x+5) +(-2x^2+4x-1)`
`-> P(x)+Q(x)=2x^2-x+5-2x^2+4x-1`
`-> P(x)+Q(x)=(2x^2-2x^2)+(-x +4x)+(5-1)`
`-> P(x)+Q(x)=3x+ 4`
