Figurate Numbers
Figurate numbers is a way of counting dots arranged in shapes like triangles and squares.
Think of it this way
Arranging blueberries on a baking tray shows figurate numbers in action. One berry is a single dot, three form a small triangle, and six form a bigger triangle with two rows. Each new row adds one more berry than the last.
Explaining figurate numbers by grade level
Put one dot down. Add two dots below it to make a triangle. Add three more dots and the triangle grows. Each row has one more dot than the last. The total dots form a number pattern. You can do the same thing with squares.