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[PLAIN]http://img411.imageshack.us/img411/6857/circle.png [Broken]

If given the diameter of the circle, as well as the required distance for AD, is it possible to find out what BC must be? It seems like this should be possible given the information, but I can't seem to find out how. Is there an equation I'm missing or something, or do I need more info?

EDIT:BC is parallel to the horizon.

Thanks...

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# Seemingly easy circle geometry can't figure it out.

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