$\tan(2x)$ was easy, as I just had to use the identity:
$$\tan(2x) = \frac{2\tan x}{1-\tan^2x}$$
I'm having a bit of trouble with $\sec(2x)$, however!
I tried squaring it and substituting a few other identities, but I either get stuck early on or go through cycles upon cycles of identity substitution which seems unnecessarily complicated.
Thanks in advance!
tan
Best Answer
Hint:
Start from $\;\cos 2x=2\cos^2x-1$ and use $1+\tan^2x=\dfrac1{\cos^2x}$.