A pentagon has five diagonals on the inside of the shape. The diagonals of any polygon can be calculated using the formula n*(n-3)/2, where "n" is the number of sides. In the case of a pentagon, which "n" will be 5, the formula as expected is equal to 5.
The number of possible diagonal lines in a polygon is proportional to the number of sides on the shape. The higher the number of sides, the more diagonals the shape will have. When diagonals are drawn in from only one vertex, the diagonal divides a polygon into triangles, which makes it easy to calculate using interior angles using trigonometry. It's important to note that diagonals are not possible in a triangle.