Now, solve .
By inspection, .
Is this the only solution?
Visually, it is difficult to tell (see Fig. 1 and Fig. 2)
However, we can prove that is the only solution:
If then from Fig. 3, we have
Area of triangle OAB < Area of circular sector OAB.
Put (1) and (2) together, we have
Therefore by (3) and (4), we conclude that
only when .