Nếu cos a = 1/ 3 , sin b = − 2 /3 thì giá trị cos ( a + b ) . cos ( a − b ) bằng

Chọn D

\(\cos \left( {a + b} \right).\cos \left( {a - b} \right) = \left( {\cos a\cos b - \sin a\sin b} \right).\left( {\cos a\cos b + \sin a\sin b} \right)\)\( = {\cos ^2}a.{\cos ^2}b - {\sin ^2}a.{\sin ^2}b\)\( = {\cos ^2}a\left( {1 - {{\sin }^2}b} \right) - \left( {1 - {{\cos }^2}a} \right).{\sin ^2}b = \frac{{ - 1}}{3}\).