proof:
\cos\ 36^\circ - \cos\ 72^\circ
= 2\sin\ 54^\circ \sin\ 18^\circ
= \dfrac{1}{2}\dfrac {\sin\ 2*54^\circ \sin\ 2*18^\circ}{\cos\ 54^\circ \cos\ 18^\circ}
= \dfrac{1}{2}\dfrac {\sin\ 108^\circ \sin\ 36^\circ}{\cos\ 54^\circ \cos\ 18^\circ}
= \dfrac{1}{2}\dfrac {\cos\ 18^\circ \sin\ 36^\circ}{\sin\ 36^\circ \cos\ 18^\circ}
= \dfrac{1}{2}
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