1987 AHSME Problems/Problem 10

Problem

How many ordered triples $(a, b, c)$ of non-zero real numbers have the property that each number is the product of the other two?

$\textbf{(A)}\ 1 \qquad \textbf{(B)}\ 2 \qquad \textbf{(C)}\ 3 \qquad \textbf{(D)}\ 4 \qquad \textbf{(E)}\ 5$

Solution

We have $ab = c$ , $bc = a$ , and $ca = b$ , so multiplying these three equations together gives $a^{2}b^{2}c^{2} = abc \implies abc(abc-1)=0$ , and as $a$ , $b$ , and $c$ are all non-zero, we cannot have $abc = 0$ , so we must have $abc = 1$ . Now substituting $bc = a$ gives $a(bc) = 1 \implies a^2 = 1 \implies a = \pm 1$ . If $a = 1$ , then the system becomes $b = c, bc = 1, c = b$ , so either $b = c = 1$ or $b = c = -1$ , giving $2$ solutions. If $a = -1$ , the system becomes $-b = c, bc = -1, -c = b$ , so $-b = c = 1$ or $b = -c = -1$ , giving another $2$ solutions. Thus the total number of solutions is $2 + 2 = 4$ , which is answer $\boxed{\text{D}}$ .

1987 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 9	Followed by Problem 11
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30
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1987 AHSME Problems/Problem 10

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