2008 AIME I Problems/Problem 13

Problem

Let

$p(x,y) = a_0 + a_1x + a_2y + a_3x^2 + a_4xy + a_5y^2 + a_6x^3 + a_7x^2y + a_8xy^2 + a_9y^3.$

Suppose that

$p(0,0) = p(1,0) = p( - 1,0) = p(0,1) = p(0, - 1)\\ = p(1,1) = p(1, - 1) = p(2,2) = 0.$

There is a point $\left(\frac {a}{c},\frac {b}{c}\right)$ for which $p\left(\frac {a}{c},\frac {b}{c}\right) = 0$ for all such polynomials, where $a$ , $b$ , and $c$ are positive integers, $a$ and $c$ are relatively prime, and $c > 1$ . Find $a + b + c$ .

Solution

$\begin{align*} p(0,0) &= a_0 = 0\\ p(1,0) &= a_0 + a_1 + a_3 + a_6 = a_1 + a_3 + a_6 = 0\\ p(-1,0) &= -a_1 + a_3 - a_6 = 0\end{align*}$

Adding the above two equations gives $a_3 = 0$ , and so we can deduce that $a_6 = -a_1$ .

Similarly, plugging in $(0,1)$ and $(0,-1)$ gives $a_5 = 0$ and $a_9 = -a_2$ . Now,

$\begin{align*}p(1,1) &= a_0 + a_1 + a_2 + a_3 + a_4 + a_5 + a_6 + a_7 + a_8 + a_9\\ &= 0 + a_1 + a_2 + 0 + a_4 + 0 - a_1 + a_7 + a_8 - a_2 = a_4 + a_7 + a_8 = 0\\ p(1,-1) &= a_0 + a_1 - a_2 + 0 - a_4 + 0 - a_1 - a_7 + a_8 + a_2\\ &= -a_4 - a_7 + a_8 = 0\end{align*}$

Therefore $a_8 = 0$ and $a_7 = -a_4$ . Finally $p(2,2) = 0 + 2a1 + 2a2 + 0 + 4a4 + 0 - 8a1 - 8a4 +0 - 8a2 = -6 a1 - 6 a2 - 4 a4 = 0$ So $3a1 + 3a2 + 2a4 = 0$ .

Now $p(x,y) = 0 + x a1 + y a2 + 0 + xy a4 + 0 - x^3 a1 - x^2 y a4 + 0 - y^3 a2$ $= x(1-x)(1+x) a1 + y(1-y)(1+y) a2 + xy (1-x) a4$ .

In order for the above to be zero, we must have $x(1-x)(1+x) = y(1-y)(1+y)$ and $x(1-x)(1+x) = 3/2 xy (1-x)$ . Canceling terms on the second equation gives us $1+x = 3/2 y \Longrightarrow x = 3/2 y - 1$ . Plugging that into the first equation and solving yieldst $x = 5/19, y = 16/19$ , and $5+16+19 = \boxed{040}$ .

2008 AIME I (Problems • Answer Key • Resources)
Preceded by Problem 12	Followed by Problem 14
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
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2008 AIME I Problems/Problem 13

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