Question

1- 1
Cos2a
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Answer 1

$1-\frac1{\cos^2\alpha}=\frac{\cos^2\alpha-1}{\cos^2\alpha}=\frac{\cos^2\alpha-\sin^2\alpha-\cos^2\alpha}{\cos^2\alpha}=\frac{-\sin^2\alpha}{\cos^2\alpha}=-tg^2\alpha$