There is a rule for this:
For any geometric figure, the number of sides is equal to the number of vertexes. Let "n" be that number.
At first, you may think that n.n is the number of diagonals but that is not the case for two reasons.
First, with such procedure, you are counting each line joining two vertexes, including the edges of the figure, which are not diagonals. So, for each vertex, you need to discard two lateral lines. That is n-2. In addition, you need to discard the same vertex
...
For any geometric figure, the number of sides is equal to the number of vertexes. Let "n" be that number.
At first, you may think that n.n is the number of diagonals but that is not the case for two reasons.
First, with such procedure, you are counting each line joining two vertexes, including the edges of the figure, which are not diagonals. So, for each vertex, you need to discard two lateral lines. That is n-2. In addition, you need to discard the same vertex
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