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3 and 4 .Determinants and Matrices
medium
Prove that the determinant $\left|\begin{array}{ccc}x & \sin \theta & \cos \theta \\ -\sin \theta & -x & 1 \\ \cos \theta & 1 & x\end{array}\right|$ is independent of $\theta$
Option A
Option B
Option C
Option D
Solution
$\Delta=\left|\begin{array}{ccc}x & \sin \theta & \cos \theta \\ -\sin \theta & -x & 1 \\ \cos \theta & 1 & x\end{array}\right|$
$=x\left(x^{2}-1\right)-\sin \theta(-x \sin \theta-\cos \theta)+\cos \theta(-\sin \theta+x \cos \theta)$
$=x^{3}-x+x \sin ^{2} \theta+\sin \theta \cos \theta-\sin \theta \cos \theta+x \cos ^{2} \theta$
$=x^{3}-x+x\left(\sin ^{2} \theta+\cos ^{2} \theta\right)$
$=x^{3}-x+x$
$\left.=x^{3} \quad \text { (Independent of } \theta\right)$
Hence, $\Delta$ is independent of $\theta$
Standard 12
Mathematics
