Maths Olympiad Prep

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Geometry Difficulty 6.5 National olympiad Find the answer

In a coordinate system, a circle with radius 77 and center is on the y-axis placed inside the parabola with equation y=x2y = x^2 , so that it just touches the parabola in two points. Determine the coordinate set for the center of the circle.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Symmetry and Circle Equation:
By symmetry, the center of the circle must lie on the y-axis, i.e., x=0x = 0. Therefore, the equation of the circle can be written as:
x2+(yn)2=49 x^2 + (y - n)^2 = 49
where nn is the y-coordinate of the center of the circle.

2. Intersection Points:
The circle touches the parabola y=x2y = x^2 at two points, which by symmetry are (a,b)(a, b) and (a,b)(-a, b). Substituting y=x2y = x^2 into the circle's equation, we get:
x2+(x2n)2=49 x^2 + (x^2 - n)^2 = 49
Simplifying, we have:
x2+(x42nx2+n2)=49 x^2 + (x^4 - 2nx^2 + n^2) = 49
x4x2(2n1)+n249=0 x^4 - x^2(2n - 1) + n^2 - 49 = 0

3. **Quadratic in yy**:
Let y=x2y = x^2. Then the equation becomes:
y+(yn)2=49 y + (y - n)^2 = 49
y+y22ny+n2=49 y + y^2 - 2ny + n^2 = 49
y2+(12n)y+n249=0 y^2 + (1 - 2n)y + n^2 - 49 = 0

4. Discriminant Condition:
Since the circle touches the parabola at exactly two points with the same y-coordinate, the quadratic equation in yy must have exactly one solution. Therefore, the discriminant of the quadratic equation must be zero:
Δ=(12n)24(n249)=0 \Delta = (1 - 2n)^2 - 4(n^2 - 49) = 0
Simplifying the discriminant:
(12n)24(n249)=0 (1 - 2n)^2 - 4(n^2 - 49) = 0
14n+4n24n2+196=0 1 - 4n + 4n^2 - 4n^2 + 196 = 0
14n+196=0 1 - 4n + 196 = 0
1974n=0 197 - 4n = 0
4n=197 4n = 197
n=1974 n = \frac{197}{4}

5. Conclusion:
Thus, the center of the circle is at:
(0,1974) (0, \frac{197}{4})

The final answer is (0,1974) \boxed{ (0, \frac{197}{4}) }

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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.