Example 11. The equation of the parabola is , and there is a circle with a radius of 1, whose center
moves on the -axis. Ask at what position
does the circle move to
when the tangents at the intersection points
of the circle and the parabola
are perpendicular to each other?
Solution
As shown in Figure 8, let the intersection point of the circle and the parabola be , and the center of the circle be point .
From the problem, the tangent line to the parabola at point must pass through the center . The equation of the tangent line is
Since point is the intersection of the tangent line and the -axis,
Solving this, we get the coordinates of the center as .
Therefore, the equation of the circle is
Next, we find the value of :
Since is the intersection point of the circle and the parabola,
By the meaning of the solution, we have
Eliminating , we get
Therefore, when the center of the circle moves to , the tangents to the circle and the parabola at the intersection point are perpendicular.
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