is an arbitrary point on the side of the triangle . is a circle tangent to the segments and in and respectively, also is tangent to the circumcircle of in . If , prove that the circumcircles of and are tangent to each other.
Iranian Mathematical Olympiad
Solution
It is obvious that the bisector of is perpendicular to and so to the (by assumption). Thus , and the circumcenter of is on the bisector of (the perpendicular bisector of ). Since the center of is on the bisector of , and is on the line connecting the centers of these two circles, it is on the bisector of , too. Hence . So is on the bisector of and hence is the incenter of . So we have:
So if we draw the ray such that and , then this ray is tangent to the circumcircles of and , and according to the above equation such ray exists. Thus the circles and are tangent in .
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