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20 15 the last question of the senior high school entrance examination in Ningde city, the last small question, who remembers what the answer is and how to do it?
It seems to be 90 degrees, 108 degrees.

90 degrees only need to prove the equilateral parallelogram and parallelism.

108 degree is more troublesome. The unknowns are set by equilateral and equilateral angles, each angle is represented by vertical and parallel, and an equation is listed by 180 degrees.

There is another situation that does not exist.