AIME 2021 I · 第 15 题

Solution 1 (Inequalities and Circles)

Note that $y=x^2-k$ is an upward-opening parabola with the vertex at $(0,-k),$ and $x=2(y-20)^2-k$ is a rightward-opening parabola with the vertex at $(-k,20).$ We consider each condition separately:

1. The two parabolas intersect at four distinct points.

By a quick sketch, we have two subconditions:

1. The point $(-k,20)$ is on or below the parabola $y=x^2-k.$

   We need $20\leq(-k)^2-k,$ from which $k\geq5.$

   Moreover, the point $(-k,20)$ is on the parabola $y=x^2-k$ when $k=5.$ We will prove that the two parabolas intersect at four distinct points at this value of $k:$

   Substituting $y=x^2-5$ into $x=2(y-20)^2-5,$ we get $x=2\left(\left(x^2-5\right)-20\right)^2-5.$ Expanding and rearranging give

$$2x^4-100x^2-x+1245=0. \hspace{20mm}(\bigstar)$$ By either the graphs of the parabolas or the Rational Root Theorem, we conclude that $x=-5$ is a root of $(\bigstar).$ So, we factor its left side:

$$(x+5)\left(2x^3-10x^2-50x+249\right)=0.$$ By either the graphs of the parabolas or Descartes' Rule of Signs, we conclude that $2x^3-10x^2-50x+249=0$ has two positive roots and one negative root such that $x\neq-5.$ So, $(\bigstar)$ has four distinct real roots, or the two parabolas intersect at four distinct points.

For Subcondition A, we deduce that $k\geq5.$

Remark for Subcondition A

Recall that if $1\leq k\leq 4,$ then the point $(-k,20)$ is above the parabola $y=x^2-k.$ It follows that for $-k\leq x\leq0:$

*   The maximum value of $y$ for the parabola $y=x^2-k$ occurs at $x=-k,$ from which $y=k^2-k\leq12.$

*   The minimum value of $y$ for the parabola $x=2(y-20)^2-k$ occurs at $x=0,$ from which $y=20-\sqrt{\frac k2}>18.$

Clearly, the parabola $x=2(y-20)^2-k$ and the left half of the parabola $y=x^2-k$ do not intersect. Therefore, the two parabolas do not intersect at four distinct points.

2. The point $(0,-k)$ is on or below the parabola $x=2(y-20)^2-k.$

   The lower half of the parabola $x=2(y-20)^2-k$ is $y=20-\sqrt{\frac{x+k}{2}}.$ We need $-k\leq20-\sqrt{\frac k2},$ which holds for all values of $k.$

   For Subcondition B, we deduce that $k$ can be any positive integer.

For Condition 1, we obtain $\boldsymbol{k\geq5}$ by taking the intersection of Subconditions A and B.

4. The four points of intersection lie on a circle with radius at most $21.$

   For equations of circles, the coefficients of $x^2$ and $y^2$ must be the same. So, we add the equation $y=x^2-k$ to half the equation $x=2(y-20)^2-k:$

$$y+\frac12x=x^2+(y-20)^2-\frac32k.$$ We expand, rearrange, and complete the squares:

$$\begin{aligned} y+\frac12x&=x^2+y^2-40y+400-\frac32k \\ \frac32k-400&=\left(x^2-\frac12x\right)+\left(y^2-41y\right) \\ \frac32k-400+\frac{1}{16}+\frac{1681}{4}&=\left(x-\frac14\right)^2+\left(y-\frac{41}{2}\right)^2. \end{aligned}$$ We need $\frac32k-400+\frac{1}{16}+\frac{1681}{4}\leq21^2,$ from which $k\leq\left\lfloor\frac{6731}{24}\right\rfloor=280.$

**For Condition 2, we obtain $\boldsymbol{k\leq280.}$**

Taking the intersection of Conditions 1 and 2 produces $5\leq k\leq280.$ Therefore, the answer is $5+280=\boxed{285}.$

~MRENTHUSIASM

Solution 2 (Translations, Inequalities, Circles)

Make the translation $y \rightarrow y+20$ to obtain $20+y=x^2-k$ and $x=2y^2-k$. Multiply the first equation by $2$ and sum, we see that $2(x^2+y^2)=3k+40+2y+x$. Completing the square gives us $\left(y- \frac{1}{2}\right)^2+\left(x - \frac{1}{4}\right)^2 = \frac{325+24k}{16}$; this explains why the two parabolas intersect at four points that lie on a circle*. For the upper bound, observe that $LHS \leq 21^2=441 \rightarrow 24k \leq 6731$, so $k \leq 280$.

For the lower bound, we need to ensure there are $4$ intersections to begin with. (Here I'm using the un-translated coordinates.) Draw up a graph, and realize that two intersections are guaranteed, on the so called "right branch" of $y=x^2-k$. As we increase the value of $k$, two more intersections appear on the "left branch":

$k=4$ does not work because the "leftmost" point of $x=2(y-20)^2-4$ is $(-4,20)$ which lies to the right of $\left(-\sqrt{24}, 20\right)$, which is on the graph $y=x^2-4$. While technically speaking this doesn't prove that there are no intersections (why?), drawing the graph should convince you that this is the case. Clearly, $k<4$ does not work.

$k=5$ does work because the two graphs intersect at $(-5,20)$, and by drawing the graph, you realize this is not a tangent point and there is in fact another intersection nearby, due to slope. Therefore, the answer is $5+280=\boxed{285}$.

• In general (assuming four intersections exist), when two conics intersect, if one conic can be written as $ax^2+by^2=f(x,y)$ and the other as $cx^2+dy^2=g(x,y)$ for polynomials $f$ and $g$ of degree at most $1$, whenever $(a,b),(c,d)$ are linearly independent (L.I.), we can combine the two equations and then complete the square to achieve $(x-p)^2+(y-q)^2=r^2$. We can also combine these two equations to form a parabola, or a hyperbola, or an ellipse. When $(a,b),(c,d)$ are not L.I., the intersection points instead lie on a line, which is a circle of radius infinity. When the two conics only have $3,2$ or $1$ intersection point(s), the statement that all these points lie on a circle is trivially true.

~Ross Gao

Solution 3 (Parabola's Properties)

Claim

Let the axes of two parabolas be perpendicular, their focal parameters be $p_1$ and $p_2$ and the distances from the foci to the point of intersection of the axes be $x_2$ and $y_1$. Suppose that these parabolas intersect at four points.

Then these points lie on the circle centered at point $(p_2, p_1)$ with radius $r = \sqrt{2(p_1^2 + p_2^2 + p_1 y_1 + p_2 x_2)}.$

Proof

Let's introduce a coordinate system with the center at the point of intersection of the axes. Let the first (red) parabola have axis $x = 0,$ focal parameter $p_1$ and focus at point $A(0, –y_1), y_1 > 0.$ Let second (blue) parabola have axis $y = 0,$ focal parameter $p_2$ and focus at point $B(–x_2,0), x_2 > 0.$ Let us denote the angle between the vector connecting the focus of the first parabola and its point and the positive direction of the ordinate axis $2\theta,$ its length $\rho_1(\theta),$ the angle between the vector connecting the focus of the second parabola and its point and the positive direction of the abscissa axis $2\phi,$ its length $\rho_2(\phi).$ Then

$$\rho_1(\theta) = \frac{p_1}{1 - \cos(2\theta)}, \rho_2(\phi) = \frac{p_2}{1 - \cos(2\phi)}.$$ Abscissa of the point of intersection is

$$\begin{aligned} x =\rho_1 \sin(2\theta) = p_1\cot\theta = \rho_2 \cos (2\phi) - x_2 = \frac{p_2}{2} (\cot^2\phi - 1)- x_2,\end{aligned}$$ $$\begin{aligned} x^2 = p_1^2 \cot ^2 \theta , 2 p_1\cot\theta = p_2 \cos^2 \phi - p_2 - 2x_2 .\end{aligned}$$ Ordinate of the point of intersection is

$$\begin{aligned} y =\rho_2 \sin 2\phi = p_2\cot\phi = \rho_1 \cos 2\theta - y_1 = \frac{p_1}{2} (\cot^2\theta - 1)- y_1,\end{aligned}$$ $$\begin{aligned} y^2 = p_2^2 \cot ^2 \phi , 2 p_2\cot\phi = p_1 \cos^2 \theta - p_1 - 2y_1 .\end{aligned}$$ The square of the distance from point of intersection to the point $(p_2, p_1)$ is

$$\begin{aligned} r^2 = (x-p_2)^2 + (y-p_1)^2 = x^2 + y^2 - 2 p_1 y - 2 p_2 x + p_1^2 + p_2^2 .\end{aligned}$$ After simple transformations, we get $r^2 = 2(p_1^2 + p_2^2 + p_1 y_1 + p_2 x_2).$

Hence, any intersection point has the same distance $r$ from the point $(p_2, p_1).$

Solution

Parameters of the parabola $y = x^2 – k$ are $p_1 = \frac{1}{2}, y_1 = 20 + k – \frac{1}{2}.$

Parameters of the parabola $\frac{x}{2} = (y – 20)^2 – \frac{k}{2}$ are $p_2 = \frac{1}{4}, x_2 = k – \frac{1}{4} \implies r^2 = 20 + \frac{3k}{2}.$

If $r \le 21, k \le \frac{842}{3},$ then integer $k \le 280.$

The vertex of the second parabola is point $(– k,20)$ can be on the parabola $y = x^2 – k$ or below the point of the parabola with the same abscissa. So

$$20 \ge (– k)^2 – k \implies 5 \le k \le 280.$$ Therefore, the answer is $5+280=\boxed{285}$.

vladimir.shelomovskii@gmail.com, vvsss

Solution 4

$\textbf{Min}:$

When $k = 4$:

$20 = x^2-4$ $x=-\sqrt{24}$

$-\sqrt{24}<-4$ $\times$

When $k = 5$:

$20 = x^2-5$ $x=-5$

$-5 = -5$ $\checkmark$

$\textbf{Max}:$

\begin{align*} \{ & \left.\begin{aligned} y &= x^2-k \\ x &= 2y^2-80y+800-k \end{aligned} \right. \end{align*}

$x = 2y^2+2y-82y+800-k$ $x = 2y^2+2x^2-2k-82y+800-k$ $2x^2+2y^2-x-82y+800-3k=0$ $x^2+y^2-\frac{x}{2}-41y+400-\frac{3}{2}k = 0$ $(x-\frac{1}{4})^2+(y-\frac{41}{2})^2+400-(\frac{1}{4})^2-(\frac{41}{2})^2 - \frac{3}{2}k = 0$ $(x-\frac{1}{4})^2+(y-\frac{41}{2})^2 = (\frac{1}{4})^2+(\frac{41}{2})^2 + \frac{3}{2}k - 400 \leq 441$ $\frac{1}{16}+\frac{1681}{4}+\frac{3}{2}k \leq 841$ $1+6724+24k \leq 13456$ $24k \leq 6631$ $k \leq \frac{6631}{24}, \quad k_{max} = 280$

Therefore the answer is $280+5 = \boxed{285}$

-cassphe

~ Edits by TThB0501

The provided solution was incorrect because of a calculation error; I have corrected the answer and fixed latex typos.

Video Solution

https://youtube.com/watch?v=7uwMm4b6OBw

~StressedPineapple