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Question
Evaluate the following:
sin 78° cos 18° − cos 78° sin 18°
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Solution
\[ \sin78^\circ\cos18^\circ - \cos 78^\circ \sin 18^\circ\]
\[ = \sin\left( 78^\circ - 18^\circ \right) \left[\text{ Using }\sin A \cos B - \cos A \sin B = \sin\left( A - B \right) \right]\]
\[ = \sin60^\circ = \frac{\sqrt{3}}{2}\]
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