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Question
Write the value of
Solution
\[\text{ Let }\Delta = \begin{vmatrix} \sin 20^\circ & - \cos 20^\circ\\\sin 70^\circ & \cos 70^\circ \end{vmatrix}\]
\[ = \sin 20^\circ \cos 70^\circ + \cos 20^\circ\sin 70^\circ\]
\[ = \sin (20^\circ + 70^\circ) \left[\text{ trignometric identity }\right]\]
\[ = \sin 90^\circ \]
\[ = 1\]
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