As shown in the figure, D is the midpoint of CB, AC is 1:6 than CB, and DB is 12cm

First, draw the line CB, define C as the starting point, B as the midpoint, and then determine the position of a, which can be divided into the following two cases:
1. When point a is to the left of point C, that is, outside line CB
∵ CD = DB = 1 / 2CB and DB = 12
∴CB=24,CD=12
And ∵ AC / CB = 1 / 6
∴AC=4
AD=AC+CD=16cm
2. When point a is to the right of point C, that is, in line segment CB,
AC = 4 as above
AD=CD-CA=12-4=8cm

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