Giải phương trình \({\tan ^2}x + {\cot ^2}x + 2\tan x + 2\cot x = 6.\)
A.\(S = \left\{ {\dfrac{\pi }{4} + k\pi ,\,\,\dfrac{{ - \pi }}{{12}} + k\pi ;\,\,\dfrac{{ - 5\pi }}{{12}} + k\pi ,\,\,k \in \mathbb{Z}} \right\}\).
B.\(S = \left\{ {-\dfrac{\pi }{4} + k\pi ,\,\,\dfrac{{ - \pi }}{{12}} + k\pi ;\,\,\dfrac{{ - 5\pi }}{{12}} + k\pi ,\,\,k \in \mathbb{Z}} \right\}\).
C.\(S = \left\{ {\dfrac{\pi }{4} + k\pi ,\,\,\dfrac{{ \pi }}{{12}} + k\pi ;\,\,\dfrac{{ - 5\pi }}{{12}} + k\pi ,\,\,k \in \mathbb{Z}} \right\}\).
D.\(S = \left\{ {\dfrac{\pi }{4} + k\pi ,\,\,\dfrac{{ \pi }}{{12}} + k\pi ;\,\,\dfrac{{ 5\pi }}{{12}} + k\pi ,\,\,k \in \mathbb{Z}} \right\}\).
