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MathNet 2011 Question 38

Math Olympiad Vault · MathNet 2011

Question 38

Let $A$ be the sum of all 10 pairwise products of the sides of a convex pentagon, $S$ be the area of the pentagon.

a) Prove that $S \le \frac{1}{5}A$.

b) Does there exist a constant $c \lt 1/5$ such that $S \le cA$?

(I. Voronovich)