For this case we have that by definition:
[tex]Sec (a) = \frac {1} {cos (a)}[/tex]
Then, we can rewrite the expression as:
[tex]Sec (120) = \frac {1} {cos (120)}[/tex]
[tex]cos (120) = cos (180-60) = - cos (60) = - \frac {1} {2}[/tex]
So:
[tex]Sec (120) = \frac {1} {- \frac {1} {2}}\\Sec (120) = - 2[/tex]
Answer:
-2
