A full circle in radians is 2π, and in degrees it's 360°. I want to convert the 72° to radians:
72° in radians = 72/360 * 2π = 2/5π
The circumference of a full circle is the number of radians times the radius. (ie., 2πr), so the JK part of the circle is:
5* 2/5π simplifies to 2π