1.

Simplify the following expression in terms of x

sq(4tan^2x+4)

I did:

sq(4(tan^2x+1))

sq(4sec^2x)

=+/- 2secx

The correct answer is 2abs(secx) and I don't understand why, so I'd appreciate it if someone would explain it to me.

2.

Write tan(arc cos(x/3)) in algebraic form

I don't remember how to do this at all, so detailed explanation on what to do would be extremely helpful.

Any help is greatly appreciated.