Question

1/cos2a - 1=tg2a
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Answer 1

$\frac{1}{cos^2a}-1=\frac{1-cos^2a}{cos^2a}=\frac{sin^2a}{cos^2a}=tg^2a\\\\\\\star \; \; sin^2a+cos^2a=1\; \; \Rightarrow \; \; sin^2a=1-cos^2a\; \; \star \\\\\star \; \; \frac{sina}{cosa}=tga\; \; \star \quad \Rightarrow \\\\\star \; \; 1+tg^2a=\frac{1}{cos^2a}\; \; \star$