I do not understand these problems;(adsbygoogle = window.adsbygoogle || []).push({});

I understand the trig basics (identities, inverses, and a bit more), but

I have'nt got a clue of where to start with these:

(sec 2 u - tan 2 u) / (cos 2 v + sin 2 v)

[ (sec y - tan y)(sec y + tan y)] / [sec y]

and this one: cos x - cos 3 x

Honestly, any help would be appreciated!! :shy:

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# Trig. expressions = help.

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