The number corresponding to a on the number axis is a, the number corresponding to B is B, satisfying | A-12 | + | B + 6 | = 0, and O is the origin If point a stays in place, point B still moves to the left at the speed of 1 unit per second, M is the midpoint of line ob, and N is the midpoint of line ab. during the movement of point B, whether the line Mn changes or not, if it changes, explain the reason, if it does not change, explain its length

The number corresponding to a on the number axis is a, the number corresponding to B is B, satisfying | A-12 | + | B + 6 | = 0, and O is the origin If point a stays in place, point B still moves to the left at the speed of 1 unit per second, M is the midpoint of line ob, and N is the midpoint of line ab. during the movement of point B, whether the line Mn changes or not, if it changes, explain the reason, if it does not change, explain its length

From the condition, A-12 = 0. B + 6 = 0, a = 12
The length of Mn is 6