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vuphong0707


a. $A= \frac {sin 3 x - sin x }{2 sin 2 x.cos x}$
b. $A= \frac {cos 3 x - cos x }{2 sin 2 x.cos x}$
b. $A= \frac {cos 3 x - cos x }{2 sin 2 x.cos x}$
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