Mathematical and Physical Journal
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Problem A. 536. (May 2011)

A. 536. The positive real numbers a, b, c, d satisfy a+b+c+d=abc+abd+acd+bcd. Prove that

{(a+b)(c+d)}+(a+d)(b+c) \ge 4\sqrt{(1+ac)(1+bd)}.

(5 pont)

Deadline expired on June 10, 2011.


4 students sent a solution.
5 points:Backhausz Tibor, Frankl Nóra, Janzer Olivér, Nagy 235 János.

Problems in Mathematics of KöMaL, May 2011