Browse · MathNet
PrintChina 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;
(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.
(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