+428 how to prove sin(18\(^\circ\)) = \(\cos (4 \cdot 18^{\circ})\) using right triangle or unit circle
+118687 Forget about that first suggestion. It was a nonsense answer.
Draw a right angle tiangle with one angle of 18degrees. Work out what the other anglw will have to be.
Call the hypotenuse h and the other two sides x and y.
Now use the trianlge to prove, or at least convince youself and anyone looking over your shoulder, that is is true :)
