Let , and be real numbers such that
(i) ,
(ii) and are defined.
Find the minimum value of .
Solution
The minimum value is .
Condition (i) can be rewritten as
If , then we have .
If , then we have .
Therefore, in any case, is a point on the unit circle on the coordinate plane. Since the steps are reversible, it can be any point on the unit circle.
Next, can be any point on the hyperbola . Therefore, we are asked to find the minimum distance between a point on the unit circle and a point on the hyperbola. In view of the geometry, since the line is an axis of symmetry of both geometric objects, the minimum distance is attained
between points on the line . By symmetry, it suffices to consider . Clearly, is a solution to and , while is a solution to and . Therefore, the minimum distance is
and we are done.
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