The first four triangle numbers are 1, 3, 6, and 10. They are called triangular because they can be arranged in dots as triangle, like so:
What will the 10th triangular number be?
There are many ways to work out a pattern. We like to start by arranging the numbers in a row to see how they relate. Underneath each number write what needs to be added, multiplied, or divided to get the next number.
Notice a pattern? The number 5 would be added next, then 6, then 7 and so on.