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secondary 3 | A Maths

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= sin (2A + A)

= sin 2A cos A + cos 2A sin A

= (2 sin A cos A) cos A + (1 - 2 sin²A) sin A

= 2 sin A cos² A + sin A - 2 sin³ A

= 2 sin A (1 - sin² A) + sin A - 2 sin³ A

= 2 sin A - 2 sin³ A + sin A - 2 sin³ A

= 3 sin A - 4 sin³ A

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