It's actually quite simple. If a pyramid is a triangular pyramid, then the parallel section and the bottom are similar triangles, and the area ratio of similar triangles is equal to the square ratio without sides. The cardinality is 2: 3. Also in the same figure (the triangle formed by the water chestnut and the base is also a similar triangle), it is concluded that the corresponding ratio of the water chestnut side is 2: 3, so the upper and lower ratio is 2: 1.

Be sure to distinguish between the upper and lower parts. The top is 2, the whole is 3, and the top and bottom are 2: 1.