Online Geometry Problem 779: Triangle, Altitude, Orthocenter, Vertex, Midpoint,
Side, Angle, 90 Degrees. Level: High School, College,
Mathematics Education

The figure below shows a triangle ABC with the
altitude CD and the orthocenter H. If F and M are the midpoints of AC and
BH, respectively, prove that the measure of the angle FDM is 90 degrees.