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Book III   Proposition 13
Then, since on the circumference of each of the circles ABDC, ACK two points A, C have been taken at random,
    the straight line joining the points will fall within each circle; [III.2]
but
    it fell within the circle ABDC and outside ACK [III.Def.3]: which is absurd.

Therefore
    a circle does not touch a circle externally at more points than one.
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