2. For what values of $a$ is the point with coordinates $(\sin a ; \sin 3 a)$ symmetric to the point with coordinates $(\cos a ; \cos 3 a)$ with respect to the line with the equation $x+y=0$?
Answer: $a= \pm \frac{\pi}{3}+2 \pi n, a=(2 n-1) \pi, n \in Z$.