Demonstração: Diagonais de um polígono d = n(n-3)/2.

The number of diagonals of a polygon is d = n(n-3)/2, where d is the number of diagonals and n is the number of sides of the polygon. In this video, we prove the formula by first observing that the number of vertices of a polygon is equal to the number of sides. Then, by counting the number of diagonals that come out of each vertex and multiplying by each vertex, noting that each diagonal is counted twice, we arrive at the result.