Similar triangles: when a triangle is formed by joining the midpoints of another equilateral triangle, its side length is half the original, so its area is 1/4 of the original (area scales with the square of the linear ratio).
Triangle DEF (midpoint triangle of ABC) has side = 1/2 of ABC's side, so area(DEF) = 1/4 area(ABC), meaning the shaded top triangle CDF also...
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