Doubly triangular number
Imagine you have a big pile of balls. You arrange them in the shape of a triangle, with one ball at the top and the next row having two balls and so on until you have a triangle. That's called a "triangular number".
Now, take another pile of balls. Arrange them in the same way, like a triangle. But this time, instead of just stacking them on top of each other, you put them in a line next to each other.
So you have two triangles of balls, one on top of the other and another triangle made up of all the balls in both piles lined up next to each other. That's called a "doubly triangular number".
A doubly triangular number is the sum of two triangular numbers. It looks like a bunch of balls arranged in a triangle with two smaller triangles on the top and bottom. It’s like having two Pyramids on top of each other.
For example, 10 is a triangular number because you can arrange 10 balls in the shape of a triangle with 4 balls on the bottom row. 6 is also a triangular number because you can arrange 6 balls in the shape of a triangle with 3 balls on the bottom row.
If we add those two numbers together (10 + 6 = 16), we get a doubly triangular number. So 16 is a doubly triangular number. It has two triangles on top of each other and is made up of 10 + 6 balls.
Now, take another pile of balls. Arrange them in the same way, like a triangle. But this time, instead of just stacking them on top of each other, you put them in a line next to each other.
So you have two triangles of balls, one on top of the other and another triangle made up of all the balls in both piles lined up next to each other. That's called a "doubly triangular number".
A doubly triangular number is the sum of two triangular numbers. It looks like a bunch of balls arranged in a triangle with two smaller triangles on the top and bottom. It’s like having two Pyramids on top of each other.
For example, 10 is a triangular number because you can arrange 10 balls in the shape of a triangle with 4 balls on the bottom row. 6 is also a triangular number because you can arrange 6 balls in the shape of a triangle with 3 balls on the bottom row.
If we add those two numbers together (10 + 6 = 16), we get a doubly triangular number. So 16 is a doubly triangular number. It has two triangles on top of each other and is made up of 10 + 6 balls.