Làm lại :
\(E=\left(a-1\right)\left(x^2+1\right)-x\left(y+1\right)+\left(x+y^2-a+1\right)\)
\(=ax^2+a-x^2-1-xy-x+x+y^2-a+1\)
\(=ax^2+a-a-x^2-1+1-xy-x+x+y^2\)
\(=ax^2-x^2-xy+y^2\)
\(E=\left(a-1\right)\left(x^2+1\right)-x\left(y+1\right)+\left(x+y^2-a+1\right)\)
\(=a\left(x^2+1\right)-\left(x^2+1\right)-xy-1+x+y^2-a+1\)
\(=ax^2+a-x^2+1-xy-1+x+y^2-a+1\)
\(=ax^2-x^2+a-a-x^2-xy-1+1+x+y^2+1\)
\(=ax^2-x^2-xy+x+y^2+1\)