1. (AUS 2) Let be a triangle. The bisector of angle meets the circumcircle of triangle in . Points and are defined similarly. Let meet the lines that bisect the two external angles at and in point . Define and similarly. If denotes the area of the polygon , prove that
Solution
1. Let denote the intersection of the three internal bisectors. Then . One way proving this is to realize that the circumcircle of is the nine-point circle of , hence it bisects , since is the orthocenter of . Another way is through noting that , which follows from , and which follows from . Hence, we obtain . Repeating this argument for the six triangles that have a vertex at and adding them up gives us . To prove , draw the three altitudes in triangle intersecting in . Let , and be the symmetric points of with respect to the sides , and , respectively. Then are points on the circumcircle of (because . Since is the midpoint of the arc , we have . Hence
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