Explicație pas cu pas:
[tex]\cos\Big( \frac{x}{2} \Big) = \pm \sqrt{ \frac{1 + \cos(x) }{2} } \\ [/tex]
[tex]\frac{3\pi}{2} < x < 2\pi \implies \frac{3\pi}{4} < \frac{x}{2} < \pi \\ [/tex]
[tex]\frac{x}{2} \rightarrow cadranul \: 2 \\ [/tex]
alegem valoarea negativă:
[tex]\cos\Big( \frac{x}{2} \Big) = - \sqrt{ \frac{1 + \cos(x) }{2} } = - \sqrt{ \frac{1 + \frac{1}{9} }{2} } = \\ = - \sqrt{ \frac{10}{18} } = - \sqrt{ \frac{5}{9} } = \bf - \frac{ \sqrt{5} }{3} [/tex]
