In a coordinate system, a circle with radius and center is on the y-axis placed inside the parabola with equation , so that it just touches the parabola in two points. Determine the coordinate set for the center of the circle.
Solution
1. Symmetry and Circle Equation:
By symmetry, the center of the circle must lie on the y-axis, i.e., . Therefore, the equation of the circle can be written as:
where is the y-coordinate of the center of the circle.
2. Intersection Points:
The circle touches the parabola at two points, which by symmetry are and . Substituting into the circle's equation, we get:
Simplifying, we have:
3. **Quadratic in **:
Let . Then the equation becomes:
4. Discriminant Condition:
Since the circle touches the parabola at exactly two points with the same y-coordinate, the quadratic equation in must have exactly one solution. Therefore, the discriminant of the quadratic equation must be zero:
Simplifying the discriminant:
5. Conclusion:
Thus, the center of the circle is at:
The final answer is