Take some triangular numbers. Multiply each one by eight and add one.There is an interesting pattern - can you spot it? Test your conjecture with some other triangular numbers.

Can you prove that the pattern continues for any triangular number?

If you are finding it tricky to construct a proof, an activity is available here which allows you to put a proof we have written in the correct order. Alternatively, look at the hint for a visual way of thinking about this problem.