The radius of a circle is the distance from the center of a circle to any point on the circle. If you place two radii end to end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius.

Pi is the relationship fo the circumference of a circle to the diameter. Thus for any circle, if you divide the circumference by the diameter, you get a value close to 3.14.

When the pentagon was built the original figure was supposed to be a circel but they over estimated the area of the land to the circumference of the circle and ended up with a pentagon.