$3$\displaystyle{ \cos x=\frac{\hspace{2} 1- {t}^{2} \hspace{2}}{\hspace{2} 1+ {t}^{2} \hspace{2}} }$

$3$\displaystyle{ \cos x={\cos }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}}- {\sin }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} }$
$3$\displaystyle{ \hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}={\cos }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} $ 1- \frac{\hspace{2} {\sin }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} \hspace{2}}{\hspace{2} {\cos }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} \hspace{2}} $ }$
$3$\displaystyle{1+{\tan }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}}=\frac{\hspace{2}1\hspace{2}}{\hspace{2} { \cos }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} \hspace{2}}}$ ‚æ‚è
$3$\displaystyle{ \hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}\hspace{5}=\frac{\hspace{2} 1- {\tan }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} \hspace{2}}{\hspace{2} 1+ {\tan }^{2}\frac{\hspace{2}x\hspace{2}}{\hspace{2}2\hspace{2}} \hspace{2}} }$