How many small triangles are in a triangular grid that has 20 units on a side. Please show me a formula to figure it out . Thanks

Answer 1

Draw a few small grids. You can see that you get

#units #triangles
2 1
3 3
4 5
5 7
...
each new row adds two triangles in its length
and as you may recall, the sum of the first n odd numbers is n^2
So, there are (n-1)^2 triangles in a grid n units on a side.

Answer 2

Don't get it