WOLFRAM|DEMONSTRATIONS PROJECT

What Triangles Are in the Triangle?

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Consider the triangular grid formed by dividing one equilateral triangle into a grid of equilateral subtriangles in such a way that there are
n
in the base. We want to know which and how many subtriangles there are within this grid. It is a very interesting exercise, involving discrete mathematics tools, to derive the formula for the total number, which is
4
3
n
+10
2
n
+4n-1+
n
(-1)
16
.