Question 27
Let $a$, $b$, $c$, $d$ be positive numbers. Prove that
$$
(a^2 + b^2 + c^2 + d^2)^2 \geq (a+b)(b+c)(c+d)(d+a).
$$
When does equality hold?
$$
(a^2 + b^2 + c^2 + d^2)^2 \geq (a+b)(b+c)(c+d)(d+a).
$$
When does equality hold?
MathNet 2015-part-01 — 191 problems (EN–VI) Question 27
Math Olympiad Vault · MathNet 2015-part-01 — 191 problems (EN–VI)