evaluate:

$\displaystyle \int_2^{4} \frac{dt}{t\sqrt{t^2-4}}$

after a trig sub I end up here:

$\displaystyle \int_2^{4} \frac{d\theta}{tan\theta}$

I'm not sure where to go from here?

This is what I was thinking:

$\displaystyle \lim_{b\rightarrow 2} \left[ \int \frac{d\theta}{tan\theta} \right]_{b}^{4}$

but not sure. Any help in finishing this is greatly appreciated!