[tex]\it \dfrac{5\pi}{12} <\dfrac{6\pi}{12} \Rightarrow \dfrac{5\pi}{12}< \dfrac{\pi}{2} \Rightarrow \dfrac{5\pi}{12}\in\Big(0,\ \dfrac{\pi}{2} \Big)\Rightarrow sin \dfrac{5\pi}{12}=cos\Big( \dfrac{\pi}{2}- \dfrac{5\pi}{12} \Big) =cos \dfrac{\pi}{12} \\ \\ \\ \left.\begin{aligned}\dfrac{11\pi}{12} < \dfrac{12\pi}{12}\Rightarrow \dfrac{11\pi}{12}<\pi\\ \\ Evident,\ \dfrac{11\pi}{12}> \dfrac{\pi}{2}\end{aligned}\right\} \Rightarrow cos \dfrac{11\pi}{12}=-cos\Big(\pi- \dfrac{11\pi}{12}\Big )\Rightarrow[/tex]
[tex]\it \Rightarrow cos\dfrac{11\pi}{12}=-cos\dfrac{\pi}{12}[/tex]
Deci, relația din enunț este echivalentă cu:
[tex]\it cos\dfrac{\pi}{12}-cos\dfrac{\pi}{12}=0\ \ (Adev\breve{a}rat\breve{a})[/tex]
