Let a = arc tan (1/2). Then tan a = 1/2. So, sec a = 2/sqrt 5tana=12.So,seca=2√5, using sec a = +-sqrt ( 1 + tan^2a)=+-sqrt ( 1+ 1/4) = +-2/sqrt 5seca=±√1+tan2a=±√1+14=±2√5. Here, tan a is positive. So, sec a is positive and = 2/sqrt 5=2√5
Now, sec(arc tan (1/2))=sec(arc sec(2/sqrt 5))=2/sqrt5sec(arctan(12))=sec(arcsec(2√5))=2√5