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'Finding Factors' printed from http://nrich.maths.org/
Patrick from Woodbridge School described
the strategy he used to find the headings:
The method I used was to reveal all of one diagonal and half of the
remaining diagonal. This is the minimal way of revealing two
expressions which share each factor, and it just fits into the
Level 3 rules. Factoring each expression then reveals a few factors
for each, and we can build up possibilities by checking for shared
factors down one column or row.