Let and be the incenter and circumcenter of a triangle , respectively. If , and , what is the area of the triangle ? Here for a line segment its length also is denoted by .
Solution
Let , be the midpoints of the sides , , respectively. Since , we see that the points , , , lie on the circle having as a diameter. In particular, the quadrilateral is inscribed in this circle, and hence we have .
Since we have from the triangle inequality that , we can take a point on the side in such a way that holds. From and , we get the fact that the triangles and are congruent, and hence that .
Thus we have , from which it follows that . Since we also have , as is the incenter of the triangle , we conclude that the triangles and are congruent. Hence, we have , and therefore, .
Denote by the foot of perpendicular line drawn from to the line . From , we see that the point lies on the side . If we let , then we get from the Pythagorean theorem that . Solving this, we get . We then have and finally, we obtain that the area of the triangle .