Q111 / 155 · WBJEE 2022 · Mathematics

Let $\Delta=\left|\begin{array}{ccc}\sin \theta \cos \phi & \sin \theta \sin \phi & \cos \theta \\ \cos \theta \cos \phi & \cos \theta \sin \phi & -\sin \theta \\ -\sin \theta \sin \phi & \sin \theta \cos \phi & 0\end{array}\right|$. Then

  • A
    $\Delta$ is independent of $\theta$
  • Correct answer
    B
    $\Delta$ is independent of $\varphi$
  • C
    $\Delta$ is a constant
  • Correct answer
    D
    $\left(\frac{d \Delta}{d \theta}\right)_{\theta-\frac{\pi}{2}}=0$

Solution

$\Delta=\left|\begin{array}{ccc}\sin \theta \cos \phi & \sin \theta \sin \phi & \cos \theta \\ \cos \theta \cos \phi & \cos \theta \sin \phi & -\sin \theta \\ -\sin \theta \sin \phi & \sin \theta \cos \phi & 0\end{array}\right|$ $\begin{aligned} \mathrm{C}_1 \longrightarrow \mathrm{C}_1-\left(\frac{\cos \phi}{\sin \phi}\right) \mathrm{C}_2 \Rightarrow \Delta=\left|\begin{array}{ccc} 0 & \sin \theta \sin \phi & \cos \theta \\ 0 & \cos \theta \sin \phi & -\sin \theta \\ -\frac{\sin \theta}{\sin \phi} & \sin \theta \cos \phi & \theta \end{array}\right| \\ &=\frac{\sin \theta}{\sin \phi} \cdot \sin \phi\left(\cos ^2 \theta+\sin ^2 \theta\right)= \sin \theta \rightarrow \text { Independent of } \phi \\ \frac{\mathrm{d} \Delta}{\mathrm{d} \theta}=\left.\cos \theta \Rightarrow \frac{\mathrm{d} \Delta}{\mathrm{d} \theta}\right|_{\theta-\frac{\pi}{2}}=0 \end{aligned}$

Hence, the correct answer is options: 'B', 'D'.

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