[tex]\sin \frac{423\pi}{11}=\sin \frac{418\pi + 5\pi}{11}=\\ \\ =\sin (38\pi + \frac{5\pi }{11})=\\ \\ =\sin (2\cdot 19\pi +\frac{5\pi}{11})=\\ \\ =\sin \frac{5\pi}{11}\\ \\ Observam \ ca: \ 0<\frac{5\pi}{11}<\frac{5\pi}{10}=\frac{\pi}{2} \Rightarrow unghiul \frac{5\pi}{11} \ e \ in \ cadranul \ I, \ prin \ urmare \ \sin \frac{5\pi}{11}>0[/tex]
