+26400 Write \(tan^2(\phi)\) in terms of \(cos(\phi)\).
\(\begin{array}{|rcll|} \hline \sin^2(\phi)+\cos^2(\phi) &=& 1 \quad & | \quad : \cos^2(\phi) \\\\ \dfrac{\sin^2(\phi)}{\cos^2(\phi)}+ \dfrac{\cos^2(\phi)}{\cos^2(\phi)} &=& \dfrac{1}{\cos^2(\phi)} \\\\ \tan^2(\phi) + 1 &=& \dfrac{1}{\cos^2(\phi)} \\ \mathbf{ \tan^2(\phi) } &=& \mathbf{\dfrac{1}{\cos^2(\phi)} -1} \\ \hline \end{array} \)
