This is a beautiful geometry problem requiring the alternate segment theorem.
Now original: ( f(x f(y) + f(x)) = y f(x) + x ). Swap x and y: ( f(y f(x) + f(y)) = x f(y) + y ). Since ( f ) is bijective and ( f(f(z))=z ), apply ( f ) to both sides of original: ( x f(y) + f(x) = f( y f(x) + x ) ) (using f(f(u))=u on RHS? Wait: original says f(A)=B ⇒ A = f(B) because f is involution? Yes, f(f(t))=t. So from ( f(x f(y)+f(x)) = y f(x) + x ), apply f: ( x f(y) + f(x) = f( y f(x) + x ) ).
The BMO2 problems are typically more complex, focusing on advanced geometry and sequence analysis. Problem 1 (Geometry - Concyclic Points) : Prove that four points related to a triangle are concyclic.
The gold standard for understanding BMO 2008