Assume,
log(secx + tanx) = t
d(log(secx + tanx)) = dt
(use chain rule to differentiate)
First differentiate,
log(secx + tanx)
Then,
(secx + tanx)

⇒ secx dx = dt
Put t and dt in the given equation we get,
⇒ ∫\(\frac{dt}{t}\)
=ln |t| + c.
But,
t = log(secx + tanx)
= ln |log(secx + tanx)| + c.