Answer
$-\dfrac {1}{\sqrt {1-\sin ^{2}\theta }}$
Work Step by Step
$sec\theta =\dfrac {1}{\cos \theta }=\dfrac {1}{-\sqrt {1-\sin ^{2}\theta }}=-\dfrac {1}{\sqrt {1-\sin ^{2}\theta }}$
(negative because In quadrant 3, $cos \theta$ is negative )
