Вычислите ctg[latex] \alpha [/latex], если cos[latex] \alpha [/latex] = -0,4 и [latex]90^{0} \ \textless \ \alpha \ \textless \ {180}^0[/latex]

[latex]cosa=-0.4 \\ a\in (II) \\ sina= \sqrt{1-cos ^{2} a} = \sqrt{1-(-0.4) ^{2} } = \sqrt{1-0.16} = \sqrt{0.84} = \\ \\ = \sqrt{ \frac{84}{100} } = \frac{2 \sqrt{21} }{10} = \frac{ \sqrt{21} }{5} \\ \\ ctga= \frac{sina}{cosa} = \frac{ \sqrt{21} }{5} *(- \frac{10}{4} )=- \frac{2 \sqrt{21} }{4} =- \frac{ \sqrt{21} }{2} [/latex]