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IMO 1991 - Official contest problems Question 1

IMO · IMO 1991 - Official contest problems

Question 1

Prove for each triangle $ABC$ the inequality
$$\frac14\lt \frac{IA\cdot IB\cdot IC}{l_A l_B l_C}\le\frac8{27},$$
where $I$ is the incenter and $l_A,l_B,l_C$ are the lengths of the angle bisectors of $ABC$.