0\; \; ,\; \; cos\frac{x}{2}<0\\\\\\cosx=2cos^2\frac{x}{2}-1=\frac{6}{13}\; ,\; \; 2cos^2\frac{x}{2}=1+\frac{6}{13}=\frac{19}{13}\; ,\; \; cos\frac{x}{2}=\pm \sqrt{\frac{19}{26}}\\\\cos\frac{x}{2}<0\; \; \Rightarrow \; \; cos\frac{x}{2}=-\sqrt{\frac{19}{26}}\\\\\\cosx=1-2sin^2\frac{x}{2}=\frac{6}{13}\; ,\; \; 2sin^2\frac{x}{2}=1-\frac{6}{13}=\frac{7}{13}\; ,\; \; sin\frac{x}{2}=\pm \sqrt{\frac{7}{26}}" alt="cosx=\frac{6}{13}\; \; ,\\\\x\in (\frac{3\pi}{2}\, ;\, 2\pi )\; \Rightarrow \; \; \frac{x}{2}\in (\frac{3\pi }{4}\, ;\, \pi)\; \; -\; 2\; chetvert\; \; \Rightarrow \; \; sin\frac{x}{2}>0\; \; ,\; \; cos\frac{x}{2}<0\\\\\\cosx=2cos^2\frac{x}{2}-1=\frac{6}{13}\; ,\; \; 2cos^2\frac{x}{2}=1+\frac{6}{13}=\frac{19}{13}\; ,\; \; cos\frac{x}{2}=\pm \sqrt{\frac{19}{26}}\\\\cos\frac{x}{2}<0\; \; \Rightarrow \; \; cos\frac{x}{2}=-\sqrt{\frac{19}{26}}\\\\\\cosx=1-2sin^2\frac{x}{2}=\frac{6}{13}\; ,\; \; 2sin^2\frac{x}{2}=1-\frac{6}{13}=\frac{7}{13}\; ,\; \; sin\frac{x}{2}=\pm \sqrt{\frac{7}{26}}" align="absmiddle" class="latex-formula">
0\; \; \to \; \; sin\frac{x}{2}=\sqrt{\frac{7}{26}}\\\\\\sin\frac{x}{2}+cos\frac{x}{2}+1,8=\sqrt{\frac{7}{26}}-\sqrt{\frac{19}{26}}+1,8\approx 0,52-0,85+1,8=1,47\approx 1,5\; ." alt="sin\frac{x}{2}>0\; \; \to \; \; sin\frac{x}{2}=\sqrt{\frac{7}{26}}\\\\\\sin\frac{x}{2}+cos\frac{x}{2}+1,8=\sqrt{\frac{7}{26}}-\sqrt{\frac{19}{26}}+1,8\approx 0,52-0,85+1,8=1,47\approx 1,5\; ." align="absmiddle" class="latex-formula">