"Common question types in Mathematical Olympiad." Qin Ke said and drew six dots with chalk:

"Suppose six people are A1, A2, A3,...,A6. If they know each other, they will be connected by a red line. If they don't know each other, they will be connected by a blue line. This becomes a picture. You only need to prove that there must be triangles of the same color in the picture.

Already."

All the students in the audience had little question marks on their faces: "???"

What are you talking about? Why do I understand everything you say, but I just don’t understand how to solve the problem? And who is Ramsey? Is there any other mathematical god in the world besides Gauss?

Qin Ke connected the dots with red and blue chalk respectively, and said: "A1 can have five sides, A1A2, A1A3,...A1A6. According to the drawer principle, there must be three sides of the same color."

Students in the audience, you look at me, what is the drawer principle? What the hell is it? Can you say something humane?

"Let's first assume that A1A2, A1A3, and A1A4 are triangles with red sides. Then if △A2A3A4 is a triangle with blue sides, then the conclusion can be proved; if △A2A3A4 has a red side, let's take A2A3 as an example. We can see that if A2A3 is red

, then △A1A2A3 is a red triangle, and the conclusion can still be proved.”

Except for Ning Qingyun and a few top mathematics students who had studied the Mathematical Olympiad, most of the students in the audience still looked confused.

"Hey, does anyone understand what Qin Ke is saying? He seems to be talking very simply, but why do I feel like I'm listening to a book from heaven and can't understand it at all?"

"What he means is that the proof has been completed?"
Chapter completed!