3 and 4 .Determinants and Matrices
easy

Evaluate $\Delta=\left|\begin{array}{ccc}0 & \sin \alpha & -\cos \alpha \\ -\sin \alpha & 0 & \sin \beta \\ \cos \alpha & -\sin \beta & 0\end{array}\right|$

A

$-2$

B

$2$

C

$3$

D

$0$

Solution

Expanding along $\mathrm{R}_{1},$ we get

$\Delta {\text{ }} = 0\left| {\begin{array}{*{20}{c}}
  0&{\sin \beta } \\ 
  { – \sin \beta }&0 
\end{array}} \right| – \sin \alpha \left| {\begin{array}{*{20}{c}}
  { – \sin \alpha }&{\sin \beta } \\ 
  {\cos \alpha }&0 
\end{array}} \right| – \cos \alpha \left| {\begin{array}{*{20}{c}}
  { – \sin \alpha }&0 \\ 
  {\cos \alpha }&{ – \sin \beta } 
\end{array}} \right|$

$=0-\sin \alpha(0-\sin \beta \cos \alpha)-\cos \alpha(\sin \alpha \sin \beta-0)$

$=\sin \alpha \sin \beta \cos \alpha-\cos \alpha \sin \alpha \sin \beta=0$

Standard 12
Mathematics

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