Stage: 4 Challenge Level: 
First prove the common difference must be even and then that it must be divisible by 3. Think about possible remainders when the middle number is divided by 3. Consider two cases.
Manipulating algebraic expressions/formulae. Inequality/inequalities. Mathematical reasoning & proof. Divisibility. Generalising. Dynamical systems. Arithmetic sequence. Modulus arithmetic. Stage 5 - Reviewed 2012. Number theory.
Published February 2002.
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