Respuesta :

HomertheGenius
u = sec x ( substitution )
u² + 2 u = 0
u ( u + 2 ) = 0
u = 0,    or   u = - 2
1/cos x = 0                    1/cos x = - 2
(not accepted )               cos x = -1/2
                                       x 1 = 2π/3,  x 2 = 4π/3