(adsbygoogle = window.adsbygoogle || []).push({}); If [tex]z=cis\theta[/tex], verify that [tex]

\tan \theta = \frac{{z - z^{ - 1} }}{{i(z + z^{ - 1} )}}

[/tex]. Use this result to prove that [tex]

\cos (2\theta ) = \frac{{1 - \tan ^2 \theta }}{{1 + \tan ^2 \theta }}

[/tex]

Ok, ive managed to verify the first equation given, but im not really sure how to use it to prove the second identity. Im really not sure where to start. If somebody could give me a hint about where to start id be very appreciative.

Thanks,

Dan.

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# Homework Help: Trig Proof

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