Greg Markowsky's Problem for Parabola
Let A and B lie to the right of the axis OO' of a parabola with vertex O, with A closer to O than B. Draw OB, and draw AA' parallel to OO'. Let C be the intersection of AA' and OB. Choose D on OO' so that CD is perpendicular to OO'. Connect BD and AD. Show that ∠ADC = ∠BDC.
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