Answer
$2 \sin{5x} \cos{2x}$
Work Step by Step
$\sin{\alpha} + \sin{\beta} =2 \sin{(\dfrac{\alpha + \beta}{2})} \cos{(\dfrac{\alpha -\beta}{2})}$
$\sin{7x} +\sin{3x} = 2 \sin{5x} \cos{2x}$
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