Вычислите. \(\cos{\frac{7\pi}{12}}\cos{\frac{5\pi}{12}}+\sin{\frac{7\pi}{12}}\sin{\frac{5\pi}{12}}\) \(\sin{\frac{9\pi}{7}}\cos{\frac{2\pi}{7}}-\cos{\frac{9\pi}{7}}\sin{\frac{2\pi}{7}}\) \(\sin{21^\circ}\sin{24^\circ}-\cos{21^\circ}\cos{24^\circ}\) \(\sin{12^\circ}\cos{78^\circ}+\cos{12^\circ}\sin{78^\circ}\) \(\frac{1}{2}\) \(1\) \(0\) \(-\frac{\sqrt{2}}{2}\) \(\frac{\sqrt{3}}{2}\) \(-1\) :