Question

Найдите cos A, если Tg A = 0,5

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Answer 1

Ответ:

$tgA=0,5\\\\1+tg^2A=\dfrac{1}{cos^2A}\ \ \ \Rightarrow \ \ \ cos^2A=\dfrac{1}{1+tg^2A}=\dfrac{1}{1+0,25}=\dfrac{1}{1,25}=\dfrac{1}{\frac{5}{4}}=\dfrac{4}{5}\\\\\\cosA=\pm \dfrac{2}{\sqrt5}=\pm \dfrac{2\sqrt5}{5}$