In triangle(ABC), D is the midpoint of AC, E is the midpoint of AB, and F is the midpoint of BC.
Construct DE, DF, and EF.
There are 4 congruent triangles: triangle(AED), triangle(EBF), triangle(DFC), and triangle(FDE).
Since all congruent triangles have equal areas, the area of triangle(ABC) is one-third the area of trapezoid(BCDE).