Question

Найдите 1/√21 sin2α, если cosα=√21/5 и 3π/2<α<2π.

Answer 1

$frac{1}{ sqrt{21}} *sin2a$
$sina = sqrt{1 ^{2} - frac{21}{25} } = frac{4}{25}$
$sin2a = frac{1}{2} (-sina)*cosa = -frac{4}{25} * frac{ sqrt{21}}{5} = -frac{4 sqrt{21} }{100} = -frac{ sqrt{21} }{25} * frac{1}{ sqrt{21} } = -[tex]sin2a = -0.04$frac{1}{25}[/tex]