Iff

Take a triangular number, multiply it by 8 and add 1. What is special about your answer? Can you prove it?
Exploring and noticing Working systematically Conjecturing and generalising Visualising and representing Reasoning, convincing and proving
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Problem

Iff printable sheet



Alison has been playing with numbers again. She started by choosing a triangular number, multiplied it by 8, and added 1. She noticed something interesting about her results...

Image
Iff
 

Try a few examples. Can you make a conjecture?

Once you've made a conjecture of your own, click below to see what Alison noticed:

 



"If $T$ is a triangular number, $8T+1$ is a square number."



Can you prove the conjecture?

You might like to have a look at this Scrambled Proof and see if you can rearrange it into the original order.

Claire thought that she could use a picture to prove this conjecture. Can you use her picture to create another proof to show that the conjecture is true?

Image
Iff



I wonder if there are any integers $k$ where $8k+1$ is a square number but $k$ is not a triangular number...

Can you prove that if $8k+1$ is a square number, $k$ must be a triangular number?

Here is a Scrambled Proof for this conjecture.

Can you use your theorem to devise a quick way to check whether the following numbers are triangular numbers?

  • 6214
  • 3655
  • 7626
  • 8656

 

The title of this problem, "Iff", is sometimes used by mathematicians as shorthand for "If and Only If", which can also be represented by the double implication arrow $\Longleftrightarrow$. To explore the difference between "If", "Only if" and "Iff", try the problem Iffy Logic.