Let be an isosceles triangle () with incenter . Circle passes through and and is tangent to . The circle intersects and circumcircle of at and , respectively. Let be the midpoint of and be the midpoint of . Prove that , and are concurrent.
Solution
Let be the midpoint of segment and be the midpoint of arc ().
We call the circumcircle of triangle , and the intersection point of and segment , . We have
So, and is the center of which gives us and . gives us . So, points and are collinear. Since , we have . Therefore
So, the lines , and are concurrent. Let be the intersection point of lines and . It suffices to show that . Since and are collinear and is the midpoint of arc , We have
Hence the result.
Looking for a route rather than an archive? The track puts 2,000
problems in a working order, from AMC 10 level to the IMO shortlist.