Respuesta :

sec x = a

sec^2 x + 2sec x = 0

a^2 + 2a = 0

a(a + 2) = 0

a = 0 or a + 2 = 0

a = -2

i.e. sec x = -2
1/cos x = -2
-2cos x = 1
cos x = -1/2

Hope I helped!