Skip to content / Đến nội dung
ThichHocToan.com

USAMO 2011 — Days 1 and 2 Question 1

USAMO · USAMO 2011 — Days 1 and 2

Question 1

Let $a$, $b$, $c$ be positive real numbers
such that $a^2+b^2+c^2+(a+b+c)^2 \le 4$. Prove that

\[ \frac{ab+1}{(a+b)^2} + \frac{bc+1}{(b+c)^2} + \frac{ca+1}{(c+a)^2} \ge 3. \]