Try this beautiful problem from Mathematical Circles book based on Triangle inequality.

Problem :

Find a point inside a convex quadrilateral such that the sum of the distances from the point to the vertices is minimal .

Key Concepts

Triangle Inequality



Check the Answer

But try the problem first…

Suggested Reading

Mathematical Circles – Chapter 6 – Triangle Inequality – Problem 6

Mathematical Circles by Dmitri Fomin , Sergey Genkin , Llia Itenberg

Try with Hints

First Hint

Do you really need a hint ? You can start thinking about the Triangle Inequality………..

Second Hint

If you have already get the idea about the main concept for this sum then you can start the problem by taking a quadrilateral ABCD with diagonals that intersect at point ‘o’.

The distance from ‘o’ to all vertices is equal.

Triangle Inequaliy image from Mathematical Circles

Here is the diagram where OA + OB + OC + OD = AC + BD – which is sum of the diagonals. We can consider this as one case …

Final Step

As a last hint you have to take another point to o’ to compare with first case

Now o’ be another point inside the quadrilateral. If we use triangle inequality here we have ,

AO’ + OC’ > AC from Triangle AO’C

BO’ + O’D > BD from Triangle BO’D

Hence AO’ + O’C + BO’ + O’D > AB + BD

Therefore its clear from this that the point that minimizes the sum of the distances is the point of intersection of diagonals.

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