Question

преобразить выражение
$cos ^{2} a frac{ tan {}^{2} a - 1}{ tan ^{2}a + 1 }$
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Answer 1

$cos^2alphadfrac{{rm tg}^2alpha-1}{{rm tg}^2alpha+1}=cos^2alpha(sin^2alpha-cos^2alpha)=-cos^2alphacos2alpha$