First of all, let u = cos x.
Then, du/dx= – sin x
or, du= – sin x.dx
or, dx= ( – 1/sinx).du
Then I = ∫(sqrt.(sec x)).dx
=∫(1/(sqrt.cos x)).dx
Putting u= cos x we get,
I = – ∫(1/(sqrt.(u(1 – u2)))).du
This integral is an elliptical integral. It wasn’t that difficult! :)