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

[latex]cosa=-0.4 \ ain (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]