For let be a triangle with side lengths , and area . Suppose that , and that is an acute triangle. Does it follow that ?
Solution
Yes, it does follow. For , let be the vertices of opposite the sides of length , respectively. We first check the case where (or or , by the same argument after relabeling). Imagine as being drawn with the base horizontal and the point above the line . We may then position so that , , and lies above the line . Then also lies inside the region bounded by the circles through centered at and . Since and are acute, the part of this region above the line lies within . In particular, the distance from to the line is less than or equal to the distance from to the line ; hence .
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