OE⊥AB？ OF⊥BC OM⊥CD ON⊥DA

Because it is a parallelogram.

So OE and OM are on the same line.

In the same way, OF and ON are on the same line.

The area formula is: bottom * height /2.

S△AOB+S△COD = AB * OE/2+CD * OM/2 = AB *(OE+OM)/2(AB = CD)

S△AOD+S△COB = BC * OF/2+AD * ON/2 = BC *(OF+ON)/2(BC = OD)

Because OE+OM is the height of the parallelogram when AB or CD is the base.

When BC or DA is the base, OF+ON is the height of parallelogram.

So S△AOB+S△COD=AB*(OE+OM)/2=S parallelogram /2.

? S△AOD+S△COB=BC*(OF+ON)/2=S parallelogram /2

So S△AOB+S△COD=S△AOD+S△COB.

2. According to the fact that the parallelogram is a central symmetric figure and the center of symmetry is the center of the parallelogram, the line where the diagonal of the well intersects with the parallelogram bisects the ground.

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