From oa=ob, oc=od, ac=bd, and the three sides are equal, it is deduced that the triangle AOC and BOD are congruent, then the angle AOC=BOD, and the angle AOB=COD is obtained by subtracting the angle BOC from both sides of the equation, because oa⊥ob and the angle COD=AOB=90 degrees, that is, oc⊥od.

Beginners should pay attention to that uppercase indicates angle, lowercase indicates edge,

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