Question

Вычислите ctg$alpha$, если cos$alpha$ = -0,4 и $90^{0} textless alpha textless {180}^0$

Answer 1

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