Question 49
Let $a, b, c, d$ be real numbers such that $a, b \le c, d$. Prove
$$
(a + b + c + d)^2 \ge 8(ac + bd).
$$
When does equality hold?
$$
(a + b + c + d)^2 \ge 8(ac + bd).
$$
When does equality hold?
MathNet 2010 — Part 02 Question 49
Math Olympiad Vault · MathNet 2010 — Part 02