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The answer to the second volume of mathematics in the second day of junior high school
LZ is talking about 13 on page 92 of the second volume of senior two mathematics, right? I happen to be doing it, too

Cut out a parallelogram with cardboard so that the intersection of its diagonals is O, fix a straight thin wooden strip lying flat on the parallelogram at 0 o'clock with a pin, and stir the thin wooden strip to make it stay at any position at will. What did you find by observing the results of several strokes? Prove your findings.

Answer: If the edge and mouth ABCD of the batten intersect at point E and point F respectively, we can find OE=OF, AE=CF, DE = BF, △ AOE △ COF.

△ DCE △ BOF, etc.