Hãy xác định kết quả sai:

$\sin \dfrac{{7\pi }}{{12}} = \sin \left( {\dfrac{\pi }{3} + \dfrac{\pi }{4}} \right)$$= \sin \dfrac{\pi }{3}\cos \dfrac{\pi }{4} + \sin \dfrac{\pi }{4}\cos \dfrac{\pi }{3}$$= \dfrac{{\sqrt 3 }}{2}.\dfrac{{\sqrt 2 }}{2} + \dfrac{1}{2}.\dfrac{{\sqrt 2 }}{2} = \dfrac{{\sqrt 6  + \sqrt 2 }}{4}$

$\cos {285^0} = \cos \left( {{{360}^0} - {{75}^0}} \right)$$= \cos {75^0} = \cos \left( {{{30}^0} + {{45}^0}} \right)$$= \cos {30^0}\cos {45^0} - \sin {30^0}\sin {45^0}$$= \dfrac{{\sqrt 3 }}{2}.\dfrac{{\sqrt 2 }}{2} - \dfrac{1}{2}.\dfrac{{\sqrt 2 }}{2}$$= \dfrac{{\sqrt 6  - \sqrt 2 }}{4}$

$\sin \dfrac{\pi }{{12}} = \sin \left( {\dfrac{\pi }{3} - \dfrac{\pi }{4}} \right)$$= \sin \dfrac{\pi }{3}\cos \dfrac{\pi }{4} - \cos \dfrac{\pi }{3}\sin \dfrac{\pi }{4}$$= \dfrac{{\sqrt 3 }}{2}.\dfrac{{\sqrt 2 }}{2} - \dfrac{1}{2}.\dfrac{{\sqrt 2 }}{2}$$= \dfrac{{\sqrt 6  - \sqrt 2 }}{4}$

$\sin \dfrac{{103\pi }}{{12}} = \sin \left( {8\pi  + \dfrac{{7\pi }}{{12}}} \right)$$= \sin \dfrac{{7\pi }}{{12}} = \dfrac{{\sqrt 6  + \sqrt 2 }}{4}$