Cos2α=? Когда ctgα=-1/3

Question

Cos2α=? Когда ctgα=-1/3

Answer 1

Task/26087071
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cos2α - ?    если  ctgα = - 1/3 .
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 cos2α = (cos²α -sin²α) /(cos²α +sin²α) = (ctg²α -1) /(ctg²α +1)  =
=(1/9 -1) / (1/9 +1) =(1-9) / (1+9)= - 0,8.   * * * ctq²α =(-1/3)² =1/9 * * *