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IMO 2013 — Complete Shortlist Question 2

IMO Shortlist · IMO 2013 — Complete Shortlist

Question 2

Prove that in any set of 2000 distinct real numbers there exist two pairs $a\gt b$ and $c\gt d$ with $a \neq c$ or $b \neq d$, such that

$$
\left|\frac{a-b}{c-d}-1\right|\lt \frac{1}{100000}
$$