The three vertices of an equilateral triangle lie respectively on the three sides and of an acute triangle .
Prove: the incenter of triangle lies inside triangle .
Solution
Below we prove a stronger result: the incenter of triangle lies inside the incircle of triangle . Let denote the distance from point to line .
Let be the incenter of and let and denote respectively the inradius of , the inradius of , and the circumradius of . Then , and what we want to prove is equivalent to proving the inequality . Assume (if the result is obvious).
Let the incircle of be tangent to sides at respectively. The lines divide the plane into 6 acute angles, each of which contains one of the points . We may assume that lies in the angle determined by lines and that contains the point (as in Figure 1). Let and be respectively the projections of onto lines and .
Since , . Since , we obtain
On the other hand, the incircle of lies inside . Hence . Similarly we obtain
Since both and are right angles, the quadrilateral is cyclic (as in Figure 1).
On this circle, arc < = arc , so arc arc . This means . From this we obtain

Fig. 1
Fig. 2