A Reuleaux triangle consists of an inscribed equilateral trinagle and 3 circular segments. The area of an equilateral triangle is given by . Where s is the length of a side.

The area of a circular segment is . There are three of those and the angle is, of course, Pi/3 or 60 degrees.

Add them up and you have your area. Now, you can use the same reasoning for a pentagon. Only you have 5 circular segments and use the area of an inscribed pentagon. The angles for the polygon will be 360/5=72.