Stage: 4 and 5 Challenge Level: 
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The idea came from a discussion between Toni Beardon and Bryan Dye (webmaster of MathsNet ) and the program has been written by Elizabeth Ayer.
Inequality/inequalities. Stage 5 - Reviewed 2012. Generalising. Number theory. Interactivities. Patterned numbers. Golden ratio. Mathematical reasoning & proof. Manipulating algebraic expressions/formulae. Quadratic equations.
Published May 2001.
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email: nrich@damtp.cam.ac.uk