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Chisinau MO p183 1979 X sin^3 a cos 3a + cos^3 a sin 3a=3/4 sin4a

Source:

March 17, 2021
trigonometryalgebra

Problem Statement

Prove the identity sin⁡3acos⁡3a+cos⁡3asin⁡3a=34sin⁡4a.\sin^3 a \cos 3a + \cos^3 a \sin 3a=\frac{3}{4}\sin 4a.
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