# Tetrahedron problem

• October 21st 2009, 09:46 PM
amoeba
Tetrahedron problem
The question is asking,
"Prove that the temperature of a tetrahedron must have at least three
distinct points on the edges or vertices of the tetrahedron with the
same value. Assume the temperature is a continuous function."

My approach would be somehow to use the mean value theorem and represent the edges as lines, but I honestly don't even know how to start this problem.
• October 22nd 2009, 10:42 AM
Opalg
Quote:

Originally Posted by amoeba
The question is asking,
"Prove that the temperature of a tetrahedron must have at least three
distinct points on the edges or vertices of the tetrahedron with the
same value. Assume the temperature is a continuous function."

My approach would be somehow to use the mean value theorem and represent the edges as lines, but I honestly don't even know how to start this problem.

I think you mean "intermediate value theorem" rather than "mean value theorem".

Suppose that the temperatures at the four vertices are $T_1,\ T_2,\ T_3,\ T_4$, with $T_1\leqslant T_2\leqslant T_3\leqslant T_4$. If $T_1 then (by the intermediate value theorem) there are points on the edges $T_1T_3$ and $T_1T_4$ where the temperature is $T_2$. If $T_1 = T_2 = T_3$ then we already have three points where the temperature is the same.

That leaves us with the case $T_1=T_2. In that case, choose $T_0$ with $T_1, and check that the edges $T_1T_3,\ T_2T_3,\ T_1T_4,\ T_2T_4$ each have a point where the temperature is $T_0$.