Cow-culus and Elegant Geometry - Numberphile

14 Nov 202221:59

TLDRThe video explores a calculus problem involving a thirsty cow and a farmer trying to minimize his walking distance to fetch water. Initially presented as a complex calculus problem, the solution is simplified using geometric reflection, transforming it into a more accessible and elegant approach. The script also touches on the broader implications of light reflection and the shortest path principle in nature, drawing a fascinating parallel between mathematical principles and everyday phenomena.


  • 📚 The video discusses a calculus problem involving a cow, a river, and a farmer, which is used as an educational example for students.
  • 🔍 The problem presented is to find the shortest path for a farmer to fetch water for a thirsty cow that is 6 km away from the river, with the farmer being 4 km down the river and 2 km away from it.
  • 📏 The solution involves using the Pythagorean theorem and calculus to minimize the total distance the farmer must walk, represented by the function f(x).
  • 🤔 The calculus approach to the problem includes taking the derivative of the distance function and finding the minimum point, which turns out to be at x = 1 km from point A.
  • 📉 The video simplifies the calculus solution by making observations that limit the range of x and by using inequalities to avoid complex calculations.
  • 🎓 A fifth grader's perspective is introduced, suggesting a simpler, non-calculus approach to solving the problem by reflecting the farmer across the river to find the shortest path.
  • 🔄 The reflection simplifies the problem by allowing the farmer to walk directly towards the cow, mimicking the path a phantom farmer on the opposite riverbank would take.
  • 📐 The geometric solution uses the concept of similar triangles to find the exact point on the riverbank where the farmer should fetch water, without the need for calculus.
  • 🌞 The video draws a connection between the farmer's path and the path of light reflecting off a surface, suggesting that both follow the shortest path.
  • 🤷‍♂️ The presenter ponders whether a real farmer without mathematical knowledge would instinctively choose the correct point on the riverbank to minimize their walk.
  • 📚 The video concludes by highlighting the elegance of the geometric solution compared to the calculus method and discusses the advantages of each approach in different scenarios.

Q & A

  • What is the main problem presented in the video script?

    -The main problem is to determine the point on the riverbank (point X) where a farmer should go to minimize the total distance walked to fetch water for a thirsty cow that is 6 kilometers away.

  • Why does the calculus solution seem daunting to the students?

    -The calculus solution appears daunting because it involves setting up a function of x, taking its derivative, and solving a complex inequality, which can be challenging without the right mathematical background or shortcuts.

  • What is the geometrical insight that simplifies the problem?

    -The geometrical insight involves reflecting the farmer across the river to create a 'phantom' farmer on the opposite bank. This simplification allows the problem to be solved by direct line segments, avoiding the complexities of calculus.

  • How does the video script use the concept of similar triangles to solve the problem?

    -The script identifies that triangle FAX is congruent to triangle F'AX and both are similar to triangle CBX. By taking the ratios of the sides of these triangles, the script shows that x equals 1, indicating the farmer should walk 1 kilometer down the river.

  • What is the significance of the farmer's path mimicking the phantom farmer's path?

    -The significance is that it proves the farmer's path will be the shortest possible, as any other path would violate the triangle inequality, making the direct path to the cow the most efficient.

  • How does the video script relate the problem to the law of reflection in physics?

    -The script draws a parallel between the farmer's shortest path to the cow and the path light takes when reflecting off a surface, suggesting that both follow the principle of taking the shortest route.

  • What alternative scenario does the script suggest to make the problem simpler?

    -The script suggests imagining the farmer and the cow on opposite sides of a straight river, which simplifies the problem to just taking a straight path to the cow.

  • What is the role of the Pythagorean theorem in the calculus solution?

    -The Pythagorean theorem is used to express the distances (hypotenuses) the farmer must travel to the river and back to the cow, which are then combined into a function to be minimized.

  • How does the script handle the complexity of the derivative calculation?

    -The script simplifies the derivative calculation by considering when the derivative is non-negative, which indicates where the function is increasing or decreasing, and thus where the minimum occurs.

  • What is the final recommendation given to the farmer in the script?

    -The final recommendation is that the farmer should walk 1 kilometer down the river from point A, dip his bucket in the water, and then go to the cow.



📚 Introducing the Farmer's Dilemma in Calculus

The script begins with a calculus problem presented to students, involving a farmer, a cow, and a river. The cow is thirsty and located 6 kilometers from the river, while the farmer is situated 4 kilometers down the river with a bucket, 2 kilometers away from the water's edge. The challenge is to determine the point on the riverbank (point X) where the farmer should fetch water to minimize the total distance walked. The problem is set up using variables and the Pythagorean theorem, aiming to find the value of x that minimizes the total distance (FX + XC). The instructor simplifies the problem by assuming the farmer will fetch water somewhere between points A and B and uses calculus to find the minimum distance, introducing the concept of derivatives to the students.


🔍 Derivative Analysis to Solve the Farmer's Problem

This paragraph delves into the calculus approach to solving the problem. The instructor transforms the problem into a function f(x) and applies the derivative to find the minimum distance. By setting up the derivative of the total distance function and analyzing when it's non-negative, the instructor simplifies the inequality and squares both sides to eliminate the square roots. After cross-multiplying and simplifying, the critical value of x is found to be when x ≥ 1. This indicates that the farmer should walk at least 1 kilometer down the river to fetch water, minimizing the total distance. The explanation highlights the power of calculus in solving optimization problems but also notes the complexity that can arise if the problem parameters change.


🎯 Reflecting on a Simpler Geometric Solution

The script then introduces a simpler geometric solution to the problem by reflecting the farmer across the river to create a 'phantom farmer' on the opposite bank. This reflection simplifies the problem, as the phantom farmer only needs to walk directly to the cow. The argument is made that the real farmer's optimal path will mirror that of the phantom farmer, as any deviation would result in a longer path, contradicting the principle of the shortest distance. The paragraph discusses the triangle inequality and how it proves that the farmer's path to point X, where he dips his bucket, will be the same as the phantom farmer's direct path to the cow.


🌟 Discovering the Connection to Light Reflection

The script makes an analogy between the farmer's path and the reflection of light. It suggests that the farmer's decision to walk to a specific point on the river to minimize distance mirrors the behavior of light reflecting off a surface, which also follows the shortest path. This comparison leads to a philosophical question about whether the law of reflection or the principle of least action (light taking the shortest path) is the fundamental axiom, hinting at their equivalence in this context.


📚 Comparing Calculus and Geometry: Problem-Solving Approaches

The final paragraph discusses the merits of both the calculus and geometric approaches to problem-solving. While the geometric solution is praised for its elegance and simplicity in this case, the calculus method is acknowledged for its applicability to more complex problems where a geometric solution may not be feasible. The instructor also teases another problem that can be solved using both methods, suggesting a future video will explore this further, and encourages viewers to explore these problem-solving techniques on their own.




Calculus is a branch of mathematics that deals with the study of change and motion, and it is often used to find the shortest distance, the rate of change, and optimization problems. In the video, calculus is used to determine the shortest path a farmer must take to reach a cow that is thirsty by the river. The script describes a calculus problem where students are challenged to find the point on the riverbank that minimizes the total distance the farmer has to walk to get water for the cow.

💡Pythagorean theorem

The Pythagorean theorem is a fundamental principle in geometry that states that in a right-angled triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. In the context of the video, the theorem is applied to calculate the distances the farmer has to travel to reach the cow, forming right triangles with the riverbank and the points A and B.


In calculus, a derivative represents the rate at which a function changes with respect to one of its variables. It is used to find the minimum or maximum values of a function. The script describes how the derivative of a function representing the total distance the farmer has to walk is taken to find the point that minimizes this distance.

💡Optimization problem

An optimization problem in mathematics involves finding the best solution among a set of available options, often by maximizing or minimizing a particular value. In the video, the farmer's problem is framed as an optimization problem where the objective is to minimize the total distance walked to reach the cow with water.


In the context of the video, reflection refers to the geometric concept of creating a mirror image of a point across a line, in this case, the river. The script introduces a clever trick where the farmer's position is reflected across the river to simplify the problem, allowing for a more intuitive solution without calculus.

💡Phantom farmer

The term 'phantom farmer' is used in the script to describe the reflected image of the actual farmer across the river. This concept is used to transform the original problem into a simpler one, where the phantom farmer can directly walk to the cow, thus helping to determine the shortest path for the real farmer.

💡Triangle inequality

The triangle inequality theorem states that for any triangle, the sum of the lengths of any two sides must be greater than or equal to the length of the remaining side. In the script, this principle is used to argue that the path mimicked by the phantom farmer is not the minimal path, leading to the conclusion that the real farmer must follow the same point as the phantom farmer to minimize the distance.


In geometry, congruence refers to the property of two figures being identical in shape and size. The script mentions that two of the triangles formed by the points A, B, and X are congruent, which helps establish the relationship between the distances and leads to the solution of the problem.

💡Similar triangles

Similar triangles are triangles that have the same shape but not necessarily the same size, with corresponding angles being equal and corresponding sides being in proportion. The script explains that the triangles formed are similar, which allows for the use of ratios to solve for the unknown distances in the problem.

💡Elegant geometry

Elegant geometry refers to the use of geometric principles and relationships to solve problems in a simple and straightforward manner. The script contrasts the elegant geometric solution, which involves reflection and similar triangles, with the more complex calculus solution, highlighting the beauty and simplicity of geometric reasoning.


A calculus problem is presented involving a cow, a river, and a farmer trying to minimize walking distance.

The problem is simplified by reflecting the farmer across the river to find the shortest path.

The farmer and the cow being on the same side of the river is identified as the complexity of the problem.

The concept of similar triangles is used to solve the problem without calculus.

The farmer should walk 1 kilometer down the river to minimize his walking distance.

The problem is analogous to the path light takes when reflecting off a surface.

The law of reflection and the principle of light following the shortest path are discussed as potentially equivalent.

A comparison is made between the elegance of the geometric solution and the utility of the calculus method for more complex problems.

The video suggests that animals, like a dog fetching a frisbee, might本能地 (instinctively) choose the most efficient path, similar to the calculus solution.

The importance of understanding the underlying principles of a problem, rather than just the calculations, is emphasized.

The video concludes with a teaser for another problem that can be solved with both calculus and a more elegant geometric method.

The transcript highlights the beauty of mathematical problem-solving and its connection to real-world phenomena.

The problem-solving process demonstrates the interplay between mathematical theory and practical, intuitive understanding.

The video encourages viewers to think critically about the assumptions and axioms underlying mathematical theorems.

The presenter uses humor and relatable examples to make complex mathematical concepts more accessible.

The transcript includes an invitation to listen to a podcast where the presenter shares personal stories and experiences.

The video description provides links to additional resources for further exploration of the topics discussed.