Mệnh đề nào sau đây sai?

\(\cos a\cos b = \frac{1}{2}\left[ {\cos \left( {a - b} \right) + \cos \left( {a + b} \right)} \right]\).

\(\sin a\cos b = \frac{1}{2}\left[ {\sin \left( {a - b} \right) - \cos \left( {a + b} \right)} \right]\).

\(\sin a\sin b = \frac{1}{2}\left[ {\cos \left( {a - b} \right) - \cos \left( {a + b} \right)} \right]\).

\(\sin a\cos b = \frac{1}{2}\left[ {\sin \left( {a - b} \right) + \sin \left( {a + b} \right)} \right]\).