For the quadrilateral , let and intersect at , and intersect at , and and intersect at . Additionally, let , and be the points of symmetry to with respect to , and respectively. Prove that one of the intersection points of and lies on the line .
Solution
Consider the statement after inversion at : Given a quadrilateral , let and intersect at , let and intersect at another point , let and intersect at another point , let be the circumcenters of respectively, prove that: one of the intersection points of and lies on .
Let be the midpoint of , let be the reflection of with respect to , and let be the other intersection point of and .
Claim 1. is the orthocenter of .
Proof. Note that both and lie on the perpendicular bisector of ,
Since , it follows that , so is the orthocenter of .
Claim 2. is the circumcenter of .
Proof. Since and , we get ; similarly we can obtain , so is a parallelogram, hence is also the midpoint of . is the perpendicular bisector of , is the perpendicular bisector of , and is the intersection point of and , so is the circumcenter of .
Claim 3. is an intersection point of and .
Proof. Since is the circumcenter of , is the antipode of on . Also, since is and is the midpoint of , we know that is the antipode of on . Hence lies on and . Similarly, we can obtain that the reflection of with respect to the midpoint of lies on , and since the midpoint of is exactly , also lies on , which completes the proof.