Problem:
Given 5 segments such that any 3 can be used to form a triangle. Show that at least one of the triangles is acute-angled.
Problem:
Given 5 segments such that any 3 can be used to form a triangle. Show that at least one of the triangles is acute-angled.
Solution:
Let the segments have lengths .
Suppose, for contradiction, that all triangles formed are obtuse-angled. Then, for any triangle with sides , the triangle is obtuse only if .
Consider the three largest segments: , , .
- For triangle with sides , , : .
- For triangle with sides , , : .
- For triangle with sides , , : .
Adding these inequalities:
Subtract from both sides:
But (since any three segments form a triangle, so the sum of the two smallest is greater than the largest), so:
But (since ), so:
But from above, (since ), which is a contradiction.
Therefore, at least one of the triangles is acute-angled.