1. The problem statement, all variables and given/known data The problem statement and proof can be found here. The proof continues after this, but I only have a question about the beginning of the proof. 2. Relevant equations NA 3. The attempt at a solution My question is simply this: Every proof I find for this problem starts assuming the inequality [itex]0≤(x-y^2)^2[/itex] and I cannot figure out why. I'm sure this is a simple matter, but any explanation would be appreciated. Thanks!