KöMaL 2016 B4773

The centre of the inscribed circle of triangle $ABC$ is $O$ , and the centre of the circumscribed circle is $K$ . Prove that the vector $\frac{\overrightarrow{AB}}{AB}+ \frac{\overrightarrow{BC}}{BC}+ \frac{\overrightarrow{CA}}{CA}$ is perpendicular to line $OK$ .