#sec(pi/4) = sqrt(2)#
Note, however, that #sqrt(2)# is not in radians. It is a ratio (not in degrees or radians); the argument of #sec# (namely #pi/4#) is the component in radians
Explanation:
#sec(pi/4)#
#color(white)("XXXX")##=1/cos(pi/4)#
#pi/4# radians is one of the standard angles with #cos(pi/4) = 1/sqrt(2)#