Skip to main content
OlympiadHQ

Browse · MathNet

Print

Silk Road Mathematics Competition

geometry

Problem

Let , , be three collinear points such that the point lies between and . Let and be parallel lines such that the points and lie on the same side of the line , and , , are not collinear. Let be the center of the circle passing through the points , , , and be the center of the circle passing through the points , , . Find all possible values of the angle , if triangles and have the same area.
Solution
If is acute, then . And if is obtuse, then . In particular, . (It can be easily seen that is not possible if .) Since triangles and are both isosceles, it follows that they are similar.
Final answer
all values strictly between 0 and 180 degrees except 90 degrees

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleAngle chasing