The midline theorem, which mentions FD+FB=FD+FN, means that F is the midpoint of ND, and because NDB is a right triangle according to the image.

C and f are the midpoint of nb and nd, that is, FC is the midline of △NBD, and FC= 1/2BD.

Because the quadrilateral ADBC is a rectangle, DB=AC.

soFC= 1/2AC

F is the midpoint of AC.

☆ P.S This question depends on what you think of the questioner's thinking. The key step is to draw a picture. If the picture is good, the problem will be solved by half.

I don't know if you understand this. This computer is not easy to type. Anyway, it is based on images combined with mathematical thinking for analysis.

I wish you progress in your studies.