The beam is sent to the plane mirror which is put on the center of this circular plane and reflected from the point $K$. When the plane mirror is rotated $10°$ in the direction of arrow, the beam is being reflected from the point $L$. Determine the lenght of the arc $KL$.
We need to have that $KL = 10°$.However, when I convert degree to radian, it seems to be $1.74778...$ which is not correct. Therefore, I didn't get how we found $\dfrac{1}{18}$.
UPDATE:
the question should be ''How much πr is the lenght of the arc KL?''. Otherwise, it won't make any sense, right? impossible to calculate the lenght without given radius. Thereby, we can try to find it in terms of $\pi r$.
Regards!
The KL arc angle is $(50 + 50) - (40 + 40) = 20$ deg
So the arc length is $\frac{1}{9}\pi r$