[url=https://www.cut-the-knot.org/pythagoras/CosCosCos.shtml]Forrás:[/url][br]Alexander Bogomolny, [u]A Quick Proof of cos(pi/7)cos(2.pi/7)cos(3.pi/7)=1/8[/u][i] from Interactive Mathematics Miscellany and Puzzles[/i][br][url=https://www.cut-the-knot.org/pythagoras/CosCosCos.shtml]https://www.cut-the-knot.org/pythagoras/CosCosCos.shtml[/url], Accessed 10 May 2018