Question 60
Points $A_1$, $A_2$, $A_3$ ... are constructed as follows: the length $OA_1$ is $4$, $\angle OA_1A_2 = 90^\circ$ and the length $A_1A_2 = 1$; then a right angle is constructed at $A_2$ to find $A_3$, and so on as shown in the diagram.
The length of $OA_{21}$ is
The length of $OA_{21}$ is