Triangles formed by line segments in a square

Question

There is a square, denoted by points A, B, C, and D. There are 30 distinct points located inside the square (call these $A_2, A_3, A_4, ... A_{31}$.

Non-intersecting segments $A_iA_j$ vertices are drawn for various pairs (i, j) with $1\le i,j\le 34$, such that the square is dissected into triangles. Assume each $A_i$ is an endpoint of at least one of the drawn segments. How many triangles are formed?

How would this look like in a diagram? How would I solve this problem?

Answer 1

We begin with Euler's formula: $v-e+f=2$. We know there are $v=34$ vertices.

We can find another formula by counting the number of face-edge pairs in two different ways: by considering (a) for each face, the number of boundary edges, and (b) for each edge, the number of neighboring faces.

The external face is bordered by $4$ edges and every other face is bordered by $3$ edges. Every edge borders $2$ distinct faces. Hence there are $2e=3(f-1)+4$ face-edge pairs in the graph.

We solve this system of linear equations to find $f$, and hence there are $f-1$ triangles.