Solution: According to the problem, A, B and C are the three vertices of a triangle, AB; BC; AC is three sides of a triangle.

So AC and BC are two sides of the same triangle.

And because d and e are the midpoints of line segments AC and BC, respectively.

So the line segment DE is one of the midlines of triangle ABC; And parallel to the AB line.

And because the length of the center line of the triangle is equal to half the length of the line segment parallel to it.

So DE=AB/2

Known ab =15 (cm);

So DE= 15/2=7.5 (cm)

Therefore, the length of the line segment DE is 7.5 cm.