• \(P+(x^2-2y^2)=x^2-y^2+3y^2-1\)

\(\Leftrightarrow P+(x^2-2y^2)=x^2+2y^2-1\)

\(\Leftrightarrow P=x^2+2y^2-1-(x^2-2y^2)\)

\(\Leftrightarrow P=x^2+2y^2-1-x^2+2y^2\)

\(\Leftrightarrow P=(x^2-x^2)+(2y^2+2y^2)-1\)

\(\Leftrightarrow P=4y^2-1\)

Vậy \(P=4y^2-1\)

• \(Q-(5x^2-xyz)=xy+2x^2-3xyz+5\)

\(\Leftrightarrow Q=xy+2x^2-3xyz+5+(5x^2-xyz)\)

\(\Leftrightarrow Q=xy+2x^2-3xyz+5+5x^2-xyz\)

\(\Leftrightarrow Q=(2x^2+5x^2)+(-3xyz-xyz)+xy+5\)

\(\Leftrightarrow Q=7x^2-4xyz+xy+5\)

Vậy \(Q=7x^2-4xyz+xy+5\)