Problem

a \sin x + b \cos(x + \theta) + b \cos(x - \theta) = d

, then the minimum value of

|\cos \theta|

is equal to a. \frac{1}{2|b|} \sqrt{d^2 - a^2} b. \frac{1}{2|a|} \sqrt{d^2 - a^2} c. \frac{1}{2|d|} \sqrt{d^2 - a^2} d. None of these