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Chisinau MO p49 1949-56 VIII-IX cos (pi/7) cos (4pi/7) cos (5pi/7) =1/8

Source:

March 15, 2021

algebratrigonometry

Problem Statement

Prove the identity:

\cos \frac{\pi}{7} \cdot \cos \frac{4\pi}{7} \cdot \cos \frac{5\pi}{7} = \frac{1}{8}