# Triangle Inequality - Mathematical Circles - Problem No. 5

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

Inequality

Geometry

Mathematical Circles - Chapter 6 - Triangle Inequality - Problem 6

Mathematical Circles by Dmitri Fomin , Sergey Genkin , Llia Itenberg

## Try with Hints

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

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.

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

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.

## Subscribe to Cheenta at Youtube

[/et_pb_text][/et_pb_column] [/et_pb_row] [/et_pb_section]

This site uses Akismet to reduce spam. Learn how your comment data is processed.

### Cheenta. Passion for Mathematics

Advanced Mathematical Science. Taught by olympians, researchers and true masters of the subject.