Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Prove it Soviet Union

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

Solution:

Let the segments have lengths abcdea \leq b \leq c \leq d \leq e.

Suppose, for contradiction, that all triangles formed are obtuse-angled. Then, for any triangle with sides xyzx \leq y \leq z, the triangle is obtuse only if z2>x2+y2z^2 > x^2 + y^2.

Consider the three largest segments: cc, dd, ee.

- For triangle with sides cc, dd, ee: e2>c2+d2e^2 > c^2 + d^2.
- For triangle with sides bb, cc, dd: d2>b2+c2d^2 > b^2 + c^2.
- For triangle with sides aa, bb, cc: c2>a2+b2c^2 > a^2 + b^2.

Adding these inequalities:

e2+d2+c2>(c2+d2)+(b2+c2)+(a2+b2)e^2 + d^2 + c^2 > (c^2 + d^2) + (b^2 + c^2) + (a^2 + b^2)

e2+d2+c2>c2+d2+b2+c2+a2+b2e^2 + d^2 + c^2 > c^2 + d^2 + b^2 + c^2 + a^2 + b^2

e2+d2+c2>a2+2b2+2c2+d2e^2 + d^2 + c^2 > a^2 + 2b^2 + 2c^2 + d^2

Subtract d2+c2d^2 + c^2 from both sides:

e2>a2+2b2+c2e^2 > a^2 + 2b^2 + c^2

But ea+be \leq a + b (since any three segments form a triangle, so the sum of the two smallest is greater than the largest), so:

e2(a+b)2=a2+2ab+b2e^2 \leq (a + b)^2 = a^2 + 2ab + b^2

But a2+2ab+b2a2+3b2a^2 + 2ab + b^2 \leq a^2 + 3b^2 (since abb2ab \leq b^2), so:

e2a2+3b2e^2 \leq a^2 + 3b^2

But from above, e2>a2+2b2+c2>a2+3b2e^2 > a^2 + 2b^2 + c^2 > a^2 + 3b^2 (since cbc \geq b), which is a contradiction.

Therefore, at least one of the triangles is acute-angled.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.