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Sixty-seven Squared

Age 16 to 18
Challenge Level Yellow star
  • Problem
  • Getting Started
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Why do this problem?

An excellent exercise in using geometric series with a fun way into the problem through experimenting with numbers and making conjectures.

Possible approach

Let the students experiment with the numbers until they spot patterns and make conjectures. If they don't see how to prove the general result you may have to ask the 'key question' and suggest using the sum of a geometric series.

Key questions

Can you write the number (k sixes followed by a 7) in terms of powers of 10 which will work for all k?

Possible extension

Try the problem Clickety Click and All the Sixes

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Binary Squares

If a number N is expressed in binary by using only 'ones,' what can you say about its square (in binary)?

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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