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.
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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