The length of 2.2. MN is equal to half the length of a.
3. If the point C is on the AB line, then the point C is in AB, and the length of MN is half that of AC+BC.
Point c is on the straight line AB, and it is possible that point c is outside AB. The length of MN at this time is half of the absolute value of (AC-BC).
Hehe, I hope I can score.