Giá trị đặc biệt nào sau đây là đúng
A. $\cos x \ne 1 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi $.
B. $\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi $.
C. $\cos x \ne - 1 \Leftrightarrow x \ne - \frac{\pi }{2} + k2\pi $.
D. $\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k2\pi $.
A. $\cos x \ne 1 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi $.
B. $\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi $.
C. $\cos x \ne - 1 \Leftrightarrow x \ne - \frac{\pi }{2} + k2\pi $.
D. $\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k2\pi $.
Chọn B.
$\cos x \ne 1 \Leftrightarrow x \ne \frac{\pi }{2} + k2\pi ,\left( {k \in \mathbb{Z}} \right)$nên A sai.
$\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi ,\left( {k \in \mathbb{Z}} \right)$ nên B đúng.
$\cos x \ne - 1 \Leftrightarrow x \ne - \pi + k2\pi ,\left( {k \in \mathbb{Z}} \right)$ nên C sai.
$\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi ,\left( {k \in \mathbb{Z}} \right)$ nên D sai.
$\cos x \ne 1 \Leftrightarrow x \ne \frac{\pi }{2} + k2\pi ,\left( {k \in \mathbb{Z}} \right)$nên A sai.
$\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi ,\left( {k \in \mathbb{Z}} \right)$ nên B đúng.
$\cos x \ne - 1 \Leftrightarrow x \ne - \pi + k2\pi ,\left( {k \in \mathbb{Z}} \right)$ nên C sai.
$\cos x \ne 0 \Leftrightarrow x \ne \frac{\pi }{2} + k\pi ,\left( {k \in \mathbb{Z}} \right)$ nên D sai.