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20 12 what are the principles and formulas used in the last additional problem of mathematics in Jiangsu college entrance examination? Don't give standard answers.
DO and DB as auxiliary lines, you can know from the meaning of the question? ODC is right angle, DAC=DCA?

OD=OA=OB

Let DAC be X.

Then DCA=X, DOC=2X, ODC=90 degrees, DCA+DOC=90 degrees, then 3X=90 degrees and X=30 degrees.

So DOC=60 degrees, OD=OB. You can know that DOB is an equilateral triangle. DB=OD=OB .

ODB=60 degrees, BDC=30 degrees =BCD,? BDC is an isosceles triangle, and BD=BC=BO.

AB = 2OA，AB=2AB .

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