2010 USAJMO Problems/Problem 4

Problem

A triangle is called a parabolic triangle if its vertices lie on a parabola $y = x^2$ . Prove that for every nonnegative integer $n$ , there is an odd number $m$ and a parabolic triangle with vertices at three distinct points with integer coordinates with area $(2^nm)^2$ .

A Small Hint

Before you read the solution, try using induction on n. (And don't step by one!)

Solution

Let the vertices of the triangle be $(a, a^2), (b, b^2), (c, c^2)$ . The area of the triangle is the absolute value of $A$ in the equation:

$A = \frac{1}{2}\det\left\vert \begin{array}{c c} b-a & c - a\\ b^2 - a^2 & c^2 - a^2 \end{array}\right\vert = \frac{(b-a)(c-a)(c-b)}{2}$

If we choose $a < b < c$ , $A > 0$ and gives the actual area. Furthermore, we clearly see that the area does not change when we subtract the same constant value from each of $a$ , $b$ and $c$ . Thus, all possible areas can be obtained with $a = 0$ , in which case $A = \frac{1}{2}bc(c-b)$ .

If a particular choice of $b$ and $c$ gives an area $A = (2^nm)^2$ , with $n$ a positive integer and $m$ a positive odd integer, then setting $b' = 4b$ , $c' = 4c$ gives an area $A' = 4^3 A = 8^2 A = (2^{n+3}m)^2$ .

Therefore, if we can find solutions for $n = 0$ , $n = 1$ and $n = 2$ , all other solutions can be generated by repeated multiplication of $b$ and $c$ by a factor of $4$ .

Setting $b=1$ and $c=2$ , we get $A=1 = (2^0\cdot1)^2$ , which yields the $n=0$ case.

Setting $b=1$ and $c=9$ , we get $A = 9\cdot4 = (2^1\cdot3)^2$ , which yields the $n=1$ case.

Setting $b=1$ and $c=5$ , we get $A=1\cdot2\cdot5 = 10$ . Multiplying these values of $b$ and $c$ by $10$ , we get $b'=10$ , $c'=50$ , $A'=10\cdot20\cdot50 = 100^2 = (2^2\cdot5^2)^2$ , which yields the $n=2$ case. This completes the construction.

Solution 2

We proceed via induction on n. Notice that we prove instead a stronger result: there exists a parabolic triangle with area $(2^nm)^2$ with two of the vertices sharing the same y-coordinate.

BASE CASE: If $n = 0$ , consider the parabolic triangle $ABC$ with $A(0, 0), B(1, 1), C(-1, 1)$ that has area $1/2 \cdot 1 \cdot 2 = 1$ , so that $n = 0$ and $m = 1$ . If $n = 1$ , let $ABC = A(5, 25), B(4, 16), C(-4, 16)$ . Because $ABC$ has area $1/2 \cdot 8 \cdot 9 = 36$ , we set $n = 1$ and $m = 3$ . If $n = 2$ , consider the triangle formed by $A(21, 441), B(3, 9), C(-3, 9)$ . It is parabolic and has area $1/2 \cdot 6 \cdot 432 = 1296 = 36^2$ , so $n = 2$ and $m = 9$ .

INDUCTIVE STEP: If $n = k$ produces parabolic triangle $ABC$ with $A(a, a^2), B(b, b^2),$ and $C(-b, b^2)$ , consider $A$ ' $B$ ' $C$ ' with vertices $A(4a, 16a^2)$ , $B(4b, 16b^2)$ , and $C(-4b, 16b^2)$ . If $ABC$ has area $(2^km)^2$ , then $A$ ' $B$ ' $C$ ' has area $(2^{k+3}m)^2$ , which is easily verified using the $1/2 \cdot\text{base} \cdot \text{height}$ formula for triangle area. This completes the inductive step for $k \implies k+3$ .

Hence, for every nonnegative integer $n$ , there exists an odd $m$ and a parabolic triangle with area $(2^nm)^2$ with two vertices sharing the same ordinate. The problem statement is a direct result of this result. -MathGenius_

Solution 3 (without induction)

First, consider triangle with vertices $(0,0)$ , $(1,1)$ , $(-1,1)$ . This has area $1$ so $n=0$ case is satisfied.

Then, consider triangle with vertices $(a,a^2), (-a,a^2), (b,b^2)$ , and set $a=2^{2n}$ and $b=2^{4n-2}+1$ . The area of this triangle is $\frac{1}{2} \cdot base \cdot height=a(b^2-a^2)$ . We have that $b^2-a^2=2^{8n-4}+2^{4n-1}+1-2^{4n}=2^{8n-4}-2^{4n-1}+1=(2^{4n-2}-1)^2$ We desire $A=a(b^2-a^2)=2^{2n}m^2$ , or $2^{4n-2}-1=m$ , and $m$ is clearly always odd for positive $n$ , completing the proof.

Solution 4

We simply need to provide an example for all $n$ that satisfies the condition, and we do so.

Let $a = 2^{2n+1}+1$ . Then consider the triangle with coordinates $(0,0), (a,a^2), (a^2,a^4) = (x_1, y_1), (x_2, y_2), (x_3, y_3)$ .

By the shoelace formula, this triangle has area $\frac{1}{2}|x_1y_2 - y_1x_2 + x_2y_3 - y_2x_3|=\frac{1}{2}|a(a^4)-a^2(a^2)|=\frac{1}{2}|a^5 - a^4| = \frac{a^4(a-1)}{2}=2^{2n}(2^{2n+1}+1)^4$ which clearly can be written in the form $(2^n \times m)^2 = 2^{2n} \times m^2$ , where $m^2 = (2^{2n+1}+1)^4$ or $m=(2^{2n+1}+1)^2$ . Now, we just have to prove that $(2^{2n+1}+1)^2$ is always odd. This is true because $2^{2n+1}$ is even (because it's a power of $2$ ), so $2^{2n+1}+1$ is odd, and since an odd number squared is odd, the whole thing is odd as well. And we are done, since we have proved that for all $n$ , we can show that there exists such a triangle by merely providing an example. $\square$

~thinker123

2010 USAJMO (Problems • Resources)
Preceded by Problem 3	Followed by Problem 5
1 • 2 • 3 • 4 • 5 • 6
All USAJMO Problems and Solutions

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