Process: Because AD/BC, EF/BC, AD//EF, and E is the midpoint of AB,

Theorem: If a set of parallel lines cut on a straight line are equal, then the line segments cut on other straight lines are also equal. A straight line parallel to one side of a triangle will bisect the third side, and a straight line parallel to the bottom will bisect the other side. The second theorem is also done: a straight line parallel to the midpoint of one side of a triangle will bisect the third side. Also known as "123 theorem"

So you can get DF=FC.