Câu 1:
`cos (3x - 30^0) = -1/2`
`<=> 3x - 30^0 = ±120^0 + k360^0`
`<=>` \(\left[ \begin{array}{l}3x = 150^0 + k360^0\\3x = -90^0 + k360^0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x = 50^0 + k120^0\\x = -30^0 + k120^0\end{array} \right.\) `(k ∈ ZZ)`
Câu 2:
`3cos^2 x - 4sin 2x + sin^2 x = 0`
`<=> 3cos^2 x - 8sin x.cos x + sin^2 x = 0`
- Với `cos x = 0`
`=> 1 = 0` (không thoả mãn)
- Với `cos x ne 0`
`=> tan^2 x - 8tan x + 3 = 0`
`<=>` \(\left[ \begin{array}{l}tan x = 4 + \sqrt{13}\\tan x = 4 - \sqrt{13}\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x = arctan (4 + \sqrt{13}) + kπ\\x = arctan (4 - \sqrt{3}) + kπ\end{array} \right.\) `(k ∈ ZZ)`
