If, in a five-sided polygon, all sides are equal and all angles also are equal, then it is called a regular pentagon. When this is not the case, it is called an irregular pentagon.
For regular pentagons, all internal angles are equal to 108 and all external angles equal 72 degrees. The sum of the five internal angles is 540 degrees.
A concave pentagon can have an internal angle that is greater than 180 degrees. In convex pentagons, no internal angle is greater than 180 degrees.Learn more about Shapes