### Iff

Prove that if n is a triangular number then 8n+1 is a square number. Prove, conversely, that if 8n+1 is a square number then n is a triangular number.

### Relative Powers

Weekly Problem 10 - 2007
The square of a number is 12 more than the number itself. The cube of the number is 9 times the number. What is the number?

### Summing Squares

Discover a way to sum square numbers by building cuboids from small cubes. Can you picture how the sequence will grow?

### Why do this problem?

This problem offers the opportunity to explore sequences generated by square numbers, which in turn can offer practice in algebraic manipulation or geometric representations to explain results. Central to the task is the idea of making and proving conjectures, and the open nature of the task offers lots of potential lines of enquiry.

### Possible approach

Show the picture of Robert's spreadsheet, or if computers are available ask learners to create it for themselves. Give them time to discuss in pairs any patterns they notice or any interesting questions that occur to them. Then bring the class together to share things they've noticed, and collect together all questions and conjectures on the board.
Possible lines of enquiry might be:
• Will square numbers only appear in columns A, D and H?
• Does the pattern of the odd squares in column A continue?
• Will the sequence continue to alternate from column A to D to A to H?
and so on.

Suggest that each pair chooses a conjecture or question that interests them, and then allow them time to explore. Stop the class while they are working, when appropriate, for mini-plenaries to share what people are working on, and to suggest ways of using algebra or geometric representations to start to justify conjectures. Learners could be given some numerical challenges: "Can you explain where $29^2$ will appear? Or $34^2$? Or $52^2$?"

Towards the end of the session, bring the class back together to share what they have been working on and to demonstrate their convincing arguments or explanations for what they have found out.

### Key questions

What patterns do you notice?
Will the patterns continue?
Can you prove it?

### Possible extension

Explore what happens when square numbers are highlighted on different grids.