Skip to main content
OlympiadHQ

Browse · MathNet

Print

VMO

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.

problem
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 .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTangentsCyclic quadrilateralsAngle chasingDistance chasingConstructions and loci