The apothem is the distance from the center to the midpoint of a side of the pentagon. So, if we draw that line and one to the corner of the pentagon(which is the radius of the circle) we get a right triangle.
The angle of this right triangle is 36 degrees = Pi/5 radians. See why?.
The length of a side of the pentagon would be
We could also get it from the radius we found by using sine: