Gọi M = cos(a + b)cos(a – b) – sin(a + b)sin(a −b) thì:

Đáp án đúng là: B

M = cos(a + b)cos(a – b) – sin(a + b)sin(a −b)

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

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

= \(\frac{1}{2} \cdot 2 \cdot \cos 2a = \cos 2a = 1 - 2{\sin ^2}a\).