Try this
Adjust the pentagon below by dragging any orange dot. By clicking on the top left command line,
you can switch it between a
regular and
irregular pentagon.

Properties of regular pentagons

Interior angle

108°

Like any regular polygon, to find the interior angle we use the formula
(180n–360)/n . For a pentagon, n=5.
See Interior Angles of a Polygon

Exterior Angle

72°

To find the exterior angle of a regular pentagon, we use the fact that the exterior angle
forms a linear pair
with the interior angle, so in general it is given by the formula
180-interior angle.
See Exterior Angles of a Polygon

The number of distinct diagonals possible from all vertices. (In general ½n(n–3) ).
In the figure above, click on "show diagonals" to see them.
See Diagonals of a Polygon

Number of triangles

3

The number of triangles created by drawing the diagonals from a given vertex. (In general n–2).
In the figure above, click on "show triangles" to see them. See Triangles of a Polygon

On the right is the well-known headquarters building for the US Department of Defense -
commonly known as "The Pentagon" due to its shape.

As you can see, it has several rings of offices inside.
These, in geometric terms, would be called concentric regular pentagons, since they share a common center point
and are symmetrical the way a regular polygon is.

It was built in 1943, has 17.5 miles (28 Km) of corridors,
and a floor area of 6,500,000 square feet (604,000 m^{2}).
In 1992, the Pentagon became a National Historic Landmark.