Take
a and
b as unit vectors from the origin (
O), representing points
A and
B on the unit circle. The angle between the vectors is
![](https://latex.codecogs.com/png.latex?\bg_white 2\theta)
. Since the distance
AB is the length of the vector
a -
b and it is less than one, the isosceles triangle
AOB must have the radii as its longest sides, meaning the angle
![](https://latex.codecogs.com/png.latex?\bg_white 2\theta)
is no more than 60 degrees. Hence, the solution should be:
![](https://latex.codecogs.com/png.latex?\bg_white 0 \le 2\theta < \frac{\pi}{3} \qquad \implies \qquad \theta \in \left[0,\ \frac{\pi}{6}\right))
Unfortunately, this isn't provided as an option. However, since
![](https://latex.codecogs.com/png.latex?\bg_white 2\theta \in [0,\ 2\pi])
, the length of vector
a -
b shortens again as
![](https://latex.codecogs.com/png.latex?\bg_white 2\theta)
exceeds pi, and so there is another set of solutions for
![](https://latex.codecogs.com/png.latex?\bg_white \frac{5\pi}{3} < 2\theta \le 2\pi \qquad \implies \qquad \theta \in \left(\frac{5\pi}{6},\ \pi\right])