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Vietnam geometry
Problem
Let be a scalene triangle. The incircle of triangle touches , and at , and respectively. The line passing through and perpendicular to cuts again at and the line passing through and perpendicular to cuts again at . The point is the midpoint of .
a) Prove that , , and are collinear.
b) Suppose that are fixed and is a moving point such that ( is a given constant). Line cut again at respectively (). Line cuts at . Prove that the perpendicular bisector of the segment passes through a fixed point.

a) Prove that , , and are collinear.
b) Suppose that are fixed and is a moving point such that ( is a given constant). Line cut again at respectively (). Line cuts at . Prove that the perpendicular bisector of the segment passes through a fixed point.
Solution
a) Since are the tangency points of with , respectively, we have . Moreover, so . Similarly, we have .
Clearly, we have or lies on the perpendicular bisector of . Furthermore, also lies on the perpendicular bisector of , therefore and are collinear.
b) Let be the midpoint of , be the intersection of with and be the reflection of over . We will show that the perpendicular bisector of always passes through . Let be the intersections of with and , respectively. We have Hence, and are concyclic, which implies . Similarly, we obtain . Since , the four points and lie on a circle with center and diameter . It follows that lies on the perpendicular bisector of the segment .
It is easy to see that the pairs of points and are symmetric with respect to , so or . From there, we have , which implies that and are symmetric with respect to . Similarly, we also have and are symmetric with respect to . Hence two perpendicular bisectors of and are symmetric with respect to . Thus, the perpendicular bisector of passes through .
Since are fixed, then is fixed. Note that remains unchanged so is fixed. Hence is fixed. So the perpendicular bisector of always passes through the fixed point .
Clearly, we have or lies on the perpendicular bisector of . Furthermore, also lies on the perpendicular bisector of , therefore and are collinear.
b) Let be the midpoint of , be the intersection of with and be the reflection of over . We will show that the perpendicular bisector of always passes through . Let be the intersections of with and , respectively. We have Hence, and are concyclic, which implies . Similarly, we obtain . Since , the four points and lie on a circle with center and diameter . It follows that lies on the perpendicular bisector of the segment .
It is easy to see that the pairs of points and are symmetric with respect to , so or . From there, we have , which implies that and are symmetric with respect to . Similarly, we also have and are symmetric with respect to . Hence two perpendicular bisectors of and are symmetric with respect to . Thus, the perpendicular bisector of passes through .
Since are fixed, then is fixed. Note that remains unchanged so is fixed. Hence is fixed. So the perpendicular bisector of always passes through the fixed point .
Techniques
Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTangentsCyclic quadrilateralsAngle chasingDistance chasingConstructions and loci