The hardest problem on the hardest test

The hardest problem on the hardest test

🎙 3Blue1Brown 👥 8.6M 📅 December 8, 2017 ⏱ 11 min 👁 16.4M 📄 science communication 🧭 2026-08-28
Available in: English (current) Français

Keywords

Putnamprobabilityspheretetrahedrongeometry

Summary

This video from 3Blue1Brown tackles a challenging problem from the Putnam competition: given four random points on a sphere, what is the probability that the tetrahedron they form contains the sphere’s center? The presenter begins by simplifying the problem to two dimensions, asking the analogous question for three points on a circle. Through a visual and intuitive approach, he shows that the probability in 2D is 1/4. He then generalizes the reasoning to 3D, using a clever reformulation: instead of choosing points directly, one can choose random diameters and then flip coins to select endpoints. This reformulation reveals that the probability in 3D is 1/8. The video emphasizes the importance of simplifying problems and finding elegant perspectives. It concludes with a related puzzle from the sponsor, Brilliant, and provides links to further resources.

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Critical Evaluation

Value of the Information & Strength of the Argument

The video provides a high-value explanation of a difficult mathematical problem. The argumentation is solid, building from a simpler 2D case to the 3D generalization. The key insight—reformulating the random selection process—is presented clearly and convincingly. The visual animations greatly aid comprehension, making the abstract concepts tangible. The reasoning is rigorous, and the conclusion is well-supported.

Scientific Rigor, Source Quality, Title Accuracy

The scientific rigor is high. The solution is mathematically correct and the explanation is precise. The video cites the Putnam archive and a written solution, providing verifiable sources. The title accurately describes the content, and the video lives up to its promise of presenting a difficult problem with an elegant solution.

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Title / Content Match

The title accurately reflects the content: the video addresses a notoriously difficult problem from the Putnam competition, and the solution is presented in an accessible yet rigorous manner.

Quality & Reliability

9/10

The video presents a rigorous mathematical solution to a Putnam problem, with clear logical steps and visual explanations. The reasoning is sound and the conclusion is correct. The channel is known for high-quality educational content.

Key Moments

Cited Sources

Concurring Sources

External References

Contribution & Novelties

The video’s original contribution lies in its pedagogical approach: it does not simply present the solution but guides the viewer through the thought process, emphasizing the strategy of simplifying the problem and reformulating the random selection. This method is broadly applicable in mathematics. The visual animations are particularly effective in conveying the geometric intuition.

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Radar Profile

The radar profile shows a very high score in information quality and reliability, with slightly lower but still strong scores in information quantity and technical level. This indicates a video that is both accurate and accessible, with a good balance of depth and breadth.

Reliability 9/10

💬 Très positif. Sur les 30 commentaires analysés, l'immense majorité exprime admiration et amusement, avec des blagues sur la difficulté du problème et des témoignages d'inspiration pour étudier les mathématiques.