Maths Olympiad Prep

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Geometry Difficulty 5.3 AIME, harder Find the answer

Example 11. The equation of the parabola is y2=2xy^{2}=2 x, and there is a circle with a radius of 1, whose center
moves on the xx-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?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

As shown in Figure 8, let the intersection point of the circle and the parabola be p(x0,y0) p\left(x_{0}, y_{0}\right) , and the center of the circle be point C \mathrm{C} .

From the problem, the tangent line to the parabola at point P \mathrm{P} must pass through the center C \mathrm{C} . The equation of the tangent line PC \mathrm{PC} is
y0y=p(x+x0). y_{0} y = p\left(x + x_{0}\right).

Since point C \mathrm{C} is the intersection of the tangent line PC \mathrm{PC} and the x x -axis,
{y0y=p(x+x0),y=0 \left\{\begin{array}{l} y_{0} y = p\left(x + x_{0}\right), \\ y = 0 \end{array}\right.

Solving this, we get the coordinates of the center C \mathrm{C} as (x0,0) \left(-x_{0}, 0\right) .
Therefore, the equation of the circle is
(x+x0)2+y2=1. \left(x + x_{0}\right)^{2} + y^{2} = 1.

Next, we find the value of x0 x_{0} :
Since P(x0,y0) \mathrm{P}\left(x_{0}, y_{0}\right) is the intersection point of the circle and the parabola,
{(x+x0)2+y2=1,y2=2x \left\{\begin{array}{l} \left(x + x_{0}\right)^{2} + y^{2} = 1, \\ y^{2} = 2 x \end{array}\right.

By the meaning of the solution, we have
{(x0+x0)2+y02=1,y02=2x0. \left\{\begin{array}{l} \left(x_{0} + x_{0}\right)^{2} + y_{0}^{2} = 1, \\ y_{0}^{2} = 2 x_{0}. \end{array}\right.

Eliminating y0 y_{0} , we get
4x02+2x01=0,x0=1±54.Since x0>0,x0=1+54. \begin{array}{c} 4 x_{0}^{2} + 2 x_{0} - 1 = 0, \\ x_{0} = \frac{-1 \pm \sqrt{5}}{4}. \\ \text{Since } x_{0} > 0, \quad \therefore x_{0} = \frac{-1 + \sqrt{5}}{4}. \end{array}

Therefore, when the center of the circle moves to (514,0) \left(-\frac{\sqrt{5} - 1}{4}, 0\right) , the tangents to the circle and the parabola at the intersection point are perpendicular.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.