Radian. An arc is one of two subsets of a circle into which any two different points belonging to it split it. Any two points A and B of the circle divide it into two parts; each of these parts is called an arc. If A and B are the ends of the diameter (i.e., the central angle of AOB is unfolded), then they define two equal arcs, called semicircles.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.