A= (x2 -y2-1) - (x2-2y2 )
A=x2 -y2 -1 - x2 + 2y2
A= (1-1) x2 + (2-1)y2 -1
A= y2 -1
theo mk là vậy đó!!!!
Theo đề bài có A+(\(x^2\)-\(2y^2\)) = \(x^2\)-\(y^2\)-1
=> A = (\(x^2\)-\(y^2\) -1)-(\(x^2\)-\(2y^2\)) = \(x^2\)-\(y^2\)-1-\(x^2\)+\(2y^2\) = (\(x^2\)-\(x^2\))+(\(-y^2\)+\(2y^2\))-1
=> A = \(y^2\)-1