Points and are located on sides and of triangle () such that the reflection of line with respect to is tangent to the circumcircle of triangle . If denotes the circumcenter of triangle , prove that the circumcircle of triangle is tangent to the circumcircle of triangle .
Solution
Let be the circumcircle of triangle . Since the reflection of line with respect to is tangent to , the reflection of with respect to is tangent to line at a point that is denoted by .
Let be the second intersection point of circumcircles of triangles and (other than ). It is claimed that circumcircles of triangles and are tangent at .
First, note that since lies on the reflection of with respect to , . Therefore,
This implies that lies on the circumcircle of triangle . On the other hand,
Therefore, lies on the circumcircle of triangle . Now it suffices to show that these two circles have the same tangent line at , or equivalently, . Since and , . However, according to the assumptions, , which completes the proof.
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