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Problem Express cos 5 x cos 3 x \cos5x\,\cos3x cos 5 x cos 3 x 1 2 ( cos 8 x − cos 2 x ) \dfrac12\bigl(\cos8x-\cos2x\bigr) 2 1 ( cos 8 x − cos 2 x ) 1 2 ( cos 8 x + sin 2 x ) \dfrac12\bigl(\cos8x+\sin2x\bigr) 2 1 ( cos 8 x + sin 2 x ) 1 2 ( cos 8 x + cos 2 x ) \dfrac12\bigl(\cos8x+\cos2x\bigr) 2 1 ( cos 8 x + cos 2 x ) 1 2 ( sin 8 x + sin 2 x ) \dfrac12\bigl(\sin8x+\sin2x\bigr) 2 1 ( sin 8 x + sin 2 x ) Browse the problem library