Skip to main content
OlympiadHQ

Browse · MathNet

Print

China Western Mathematical Olympiad

China geometry

Problem

In , , the inscribed circle touches , , at points , and respectively. is a point on arc (not containing ). Line intersects the circle at another point , and lines , meet line at , respectively. Prove that

(1) , , , are concyclic;

Solution
Proof

(1) From the given condition, , so and thus , , , are concyclic.

(2) By the sine law, and the fact that , , , are concyclic, we have Together with , the proposition is proven.

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsTangentsAngle chasingTrigonometry