Stage: 3 Challenge Level: 
Solve the system of equations
$$\begin{eqnarray} xy &=& 1 \\ yz &=& 4 \\ zx &=& 9 \end{eqnarray}$$.
Manipulating algebraic expressions/formulae. Algebra - generally. Integers. Simultaneous equations. Other equations. Diophantine equations. Generalising. Mathematical reasoning & proof. Games. Working systematically.
Published October 1999.
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