2. For what values of $a$ does the equation $\sin 5 a \cdot \cos x - \cos (x + 4 a) = 0$ have two solutions $x_{1}$ and $x_{2}$, such that $x_{1} - x_{2} \neq \pi k, k \in Z$?
Answer: $a=\frac{\pi(4 t+1)}{2}, t \in Z$.