19. If $x_{1}, x_{2}$ are the roots of the equation $x^{2}-x \sin \frac{4 \pi}{7}$ $+\cos \frac{4 \pi}{7}=0$, then $\operatorname{arctg} x_{1}$ $+\operatorname{arctg} x_{2}$ is $\qquad$ -
19. $\frac{3 \pi}{14}$