Biến đổi tổng thành tích A = sina + sinb + sin(a + b)

A = sina + sinb + sin(a + b)

\(A = 2\sin \frac{{a + b}}{2}\cos \frac{{a - b}}{2} + 2\sin \frac{{a + b}}{2}\cos \frac{{a + b}}{2}\)

\(A = 2\sin \frac{{a + b}}{2}\left( {\cos \frac{{a - b}}{2} + \cos \frac{{a + b}}{2}} \right)\)

\(A = 2\sin \frac{{a + b}}{2}.2\cos \frac{a}{2}\cos \frac{b}{2}\)

\(A = 4\sin \frac{{a + b}}{2}.\cos \frac{a}{2}.\cos \frac{b}{2}\).