Segment addition and midpoints.

Explanation:

AC = 13 and BC = 8

Those two facts must mean AB = AC-BC = 13 - 8 = 5.

Similarly, CD = BD - BC = 12 - 8 = 4

So,

AD = AB + BC + CD

AD = 5 + 8 + 4

AD = 17

Another way to approach this problem would be to say

AD = AC + BD - BC

AD = 13 + 12 - 8

AD = 17

This works because when we add up AC with BD, we're double counting the portion from B to C. So this is why we subtract off BC to correct for this overcounting so to speak.

Segment AD is 17.

Step-by-step explanation:

We know AC is 13, BC is 8, and BD is 12. We need the lengths of AB and CD.

To get AB's length, subtract BC's length from AC's length. We should get 5 for the length of AB (13 - 8 = 5).

To get CD's length, subtract BD's length from BC's length. We should get 4 for the length of CD. (12 - 8 = 4)

Now to add AB, BC, and CD. Add 5, 8, and 4 to get 17. (13 + 4 = 17)

The length of Segment AD is 17.