Stage: 3 Challenge Level: 
Solve the system of equations
$$\begin{eqnarray} xy &=& 1 \\ yz &=& 4 \\ zx &=& 9 \end{eqnarray}$$.
smartphone. Creating expressions/formulae. Working systematically. Integers. Manipulating algebraic expressions/formulae. Generalising. Simultaneous equations. Using symbols. Solving equations graphically. Diophantine equations.
Published October 1999.
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