theta := s/r radians

Radians are dimension-less units.

When you draw a circle with any radius around an angle, the angle’s measure in radians is simply the ratio of the arclength to the radius.  This ratio is the same for any circle drawn around the vertex of the angle.

If a circle is drawn around an angle where the center of the circle is the vertex of the angle, we call the angle a CENTRAL ANGLE.

Pi = 3.1415... is a number

Pi happens to be the ratio of a circle’s circumference to it’s diameter:  C/D = 2*Pi*r/2*r = Pi