Question

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

$\cos^2\alpha\dfrac{{\rm tg}^2\alpha-1}{{\rm tg}^2\alpha+1}=\cos^2\alpha(\sin^2\alpha-\cos^2\alpha)=-\cos^2\alpha\cos2\alpha$