Đáp án: Chọn `A`
Giải thích các bước giải:
`2sin x + 1 = 0`
`<=> sin x = -1/2`
`<=>` \(\left[ \begin{array}{l}x = -\dfrac{π}{6} + k2π\\x = \dfrac{7π}{6} + k2π\end{array} \right.\)
Có:
`(-3π)/2 ≤ x ≤ 10π`
`<=>` \(\left[ \begin{array}{l}\dfrac{-3π}{2} ≤ -\dfrac{π}{6} + k2π ≤ 10π\\\dfrac{-3π}{2} ≤ \dfrac{7π}{6} + k2π ≤ 10π\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}\dfrac{-4π}{3} ≤ k2π ≤ \dfrac{61π}{6}\\\dfrac{-8π}{3} ≤ k2π ≤ \dfrac{53π}{6}\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}\dfrac{-2}{3} ≤ k ≤ \dfrac{61}{12}\\\dfrac{-4}{3} ≤ k ≤ \dfrac{53}{12}\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}k = 0; 1; 2; 3; 4; 5\\k = -1; 0; 1; 2; 3; 4\end{array} \right.\)
`=>` Có 12 giá trị `k`
`=>` Chọn `A`
