Browse · MathNet
PrintBxMO Team Selection Test
Netherlands geometry
Problem
Let be an acute triangle, and let be the foot of the altitude from . The circle with centre passing through intersects the circumcircle of triangle in and , in such a way that the order of the points on this circumcircle is: , , , , . Show that .

Solution
As the radius is perpendicular to , the line is tangent to the circumcircle of . By the inscribed angle theorem (tangent case), we have . Moreover, the quadrilateral is cyclic, so . By the sum of angles in , we have . As , we obtain .
Techniques
TangentsCyclic quadrilateralsAngle chasing