Answer
$y'=sec(tanx)tan(tanx)sec^2(x)$
Work Step by Step
Find y and u:
$y=sec(u)$
$u=tan(x)$
First, find the derivative of y and u. Don't forget to apply chain rule for y':
$y'=sec(u)tan(u)\times u'$
$u'=sec^2(x)$
Then plug in u and u' into y':
$y'=sec(tanx)tan(tanx)sec^2(x)$
