Numbers that can be represented by a triangular
arrangement of dots are called triangular numbers. The
first four triangular numbers are shown.
Describe the pattern in the first four numbers.
Then list the next three triangular numbers.
Each extension to the next triangle is one more dot than the previous, since the first "triangle" was only one dot and since the last triangle shown has layers, the next triangle will be 10 + 5, it will have 15 dot.
