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Forrás: https://answers.yahoo.com/question/index?qid=20160313125122AAcqAnp
Suppose that circles A and B are externally tangent. Let DE is a common external tangent of the two circles. If a circle C is tangent to both circles and the common external tangent, and a is the radius of circle A, b is the radius of circle B, and c is the radius of circle C, prove that 1/sqrt(c) = 1/sqrt(a) + 1/sqrt(b)
Let C1 and C2 be two circles that intersect at two points A and B. Let P be the point diametrically opposite A on C1, and let Q be the point diametrically opposite A on C2. Are P, B, and Q collinear? Prove that they are or are not. Do a geometric proof AND an analytic geometry proof (a proof using general coordinates).