Tolong banget,ga ngerti gimana caranya

[tex] \cos( \alpha ) = \frac{1}{2} \\ dimana \: \alpha \: dikuadran \: 4 \: (270 < \alpha < 360) \\ \sin( \alpha ) = \sqrt{1 - {cos}^{2} \alpha } = \sqrt{1 - { (\frac{1}{2} )}^{2} } = \sqrt{1 - \frac{1}{4} } = - \frac{ \sqrt{3} }{2} \\ \tan( \alpha ) = \frac{ \sin( \alpha ) }{ \cos( \alpha ) } = - \sqrt{3} [/tex]