Ta co:\(x^2-y=y^2-x\)
\(\Leftrightarrow\left(x-y\right)\left(x+y+1\right)=0\)
\(\Leftrightarrow\orbr{\begin{cases}x=y\\x+y=-1\end{cases}}\)
TH:\(x=y\left(l\right)\)(Vi x,y la 2 so khac nhau)
TH:\(x+y=-1\)
Ta co:\(A=\left(x+y\right)^2-3\left(x+y\right)=1+3=4\)