\begin{array}{c} \color{#FFFFFFff}{F}\color{#D0E4FFff}{u}\color{#A4C8FFff}{r}\color{#78ACFFff}{i}\color{#4C90FFff}{n}\color{#2064FFff}{a} \color{#0040A4ff}{F}\color{#2064FFff}{o}\color{#4C90FFff}{r}\color{#78ACFFff}{c}\color{#A4C8FFff}{a}\color{#D0E4FFff}{l}\color{#FFFFFFff}{o}\color{#D0E4FFff}{s} \end{array}

Bài `9:`

`a,A+x^2-y^2=x^2-2y^2 +3xy-2`

`A=x^2-2y^2+3xy-2-x^2+y^2`

`A=(x^2-x^2)+(-2y^2+y^2)+3xy-2`

`A=-y^2 +3xy-2`

`b,B-(5x^2-2xyz)=2x^2+2xyz+1`

`B=2x^2+2xyz+1+5x^2-2xyz`

`B=7x^2+1`