Given, #sec a/tan a = 1/3#
#rArr (1/cos a)/(sin a/cos a) = 1/3#
#rArr 1/cos a. cos a/sin a = 1/3#
#rArr 1/sin a = 1/3#
#rArr cosec a = 1/3# [ squaring both sides]
#rArr cosec^2 a = 1/9#
#rArr 1 + cot^2 a = 1/9# [# we know, 1 + cot^2 a = cosec^2 a#]
#rArr cot^2 a = 1/9-1#
#rArr cot^2 a = -8/9#
#rArr cot a = sqrt (-8/9)#