Ta đặt:
\(\left\{ \matrix{ u = a - b \hfill \cr v = a + b \hfill \cr} \right. \Rightarrow \left\{ \matrix{ a = {{u + v} \over 2} \hfill \cr b = {{v - u} \over 2} \hfill \cr} \right.\)
\(\eqalign{
& + )\,\,\,\cos u + \cos v = \cos (a - b) + \,\cos (a + b) \cr
& = \cos a\cos b = \cos {{u + v} \over 2}.cos{{v - u} \over 2} \cr
& + )\,\sin u + \sin v = \sin (a - b) + \sin (a + b) \cr
& = \sin a\cos b = \sin {{u + v} \over 2}.cos{{v - u} \over 2} \cr} \)
