Originally Posted by Bostonian
[quote=ultramarina]

x + y = (x^2- y^2)/(x-y)

This is a consequence of x^2 - y^2 = (x+y) * (x-y), which is an important formula which I guess would be taught in Algebra II. She has probably has some math talent if she is deriving the formula she did while in Algebra I.

It would indeed be impressive if she self-derived, but in AOPS this is Algebra I and not II. Do check what they are doing in their math class.