(minor variant of a problem due to Roy Barbara)
Let be three positive real numbers.
Find necessary and sufficient conditions on for there to exist an interior point in the equilateral triangle with unit side, such that .
I have a solution to this, but as I have not looked it up so I cannot tell if it is the originator's solution but as it is not as neat as I would like it is probably clumsy compared to the best solution, so lets see what we can do
The idea is to use barycentric coordinates: for any point in the plane, there is a unique triplet such that and .
Given these coordinates, lies in the triangle if, and only if the three numbers , , are positive (or zero, corresponding to bounderies).
It is easy to express in terms of . We have , hence .
Because is equilateral with unit sides, we conclude .
By circular permutation of letters, we get similar expressions for and . Thus, .
What we need in fine is expressions for in terms of . This can be laborious, but I found a soft way to write it. We have , hence .
Again by circular permutation of the letters, we have . We deduce .
Finally, we have . And, similarly, and .
Remembering what I said first about barycentric coordinates, the conclusion is then straightforward: lies inside the triangle if, and only if , and .
If the triangle had side , it would suffice to replace 1 by in the conditions, making them more "homogeneous".