# How to lie using visual proofs

TLDRIn this educational video, the speaker debunks three visually persuasive but incorrect mathematical proofs, illustrating the importance of rigor in mathematical reasoning. The first proof incorrectly calculates the surface area of a sphere, while the second attempts to prove that pi equals 4 using a sequence of approximating curves. The third, a Euclidean-style proof, falsely claims all triangles are isosceles. Each example highlights the pitfalls of relying on visual intuition and the necessity for careful analysis and proof in mathematics.

### Takeaways

- 📚 The video discusses three fake mathematical proofs to demonstrate the importance of rigor in mathematical reasoning.
- 🌐 The first proof incorrectly calculates the surface area of a sphere by approximating it as a rectangle, leading to an erroneous formula.
- 🔍 The error in the sphere proof is highlighted by comparing it with a correct method for finding the area of a circle using pizza slices, emphasizing the importance of proper geometric transformations.
- 📏 The second proof attempts to show that pi equals 4 by approximating a circle with a square and then a sequence of curves, which is flawed due to incorrect assumptions about the limit of the curves' perimeters.
- 📐 The third proof claims all triangles are isosceles through a Euclidean-style argument, which is subtly incorrect due to a misidentified angle bisector intersection point.
- 🧐 The video emphasizes the need for critical thinking and the pitfalls of relying solely on visual intuition without proper mathematical justification.
- 🔺 The difference between the incorrect sphere proof and the correct circle proof lies in the linear vs. non-linear transformation of the geometric shapes involved.
- 🤔 The concept of limits is explored, showing that the limit of a sequence of curves does not necessarily preserve the properties of the original sequence, such as perimeter in the case of the circle approximation.
- 📉 The video uses the example of a rearrangement puzzle to illustrate how visual tricks can lead to incorrect conclusions about area and shape.
- 🌀 Gaussian curvature is mentioned as a key concept that differentiates the geometry of a sphere from flat space, affecting the validity of geometric proofs.
- ⚠️ The video concludes with a reminder that while visual proofs can be helpful, they must be supplemented with rigorous mathematical proof to avoid fallacies.

### Q & A

### What is the first fake proof in the script about?

-The first fake proof is about a formula for the surface area of a sphere, which incorrectly assumes that subdividing the sphere into vertical slices and unraveling them would lead to a rectangle whose area can be calculated as pi squared times r squared, instead of the correct formula, which is 4 pi r squared.

### What is the fundamental flaw in the sphere surface area proof?

-The fundamental flaw is that the proof assumes that the edges of the unraveled slices would form a perfect rectangle, which is not the case due to the curvature of the sphere. The slices should bulge outward, and there would be an overlap that persists even with finer subdivisions, leading to an incorrect surface area calculation.

### How does the script explain the concept of a limit in the context of approximating a circle with a sequence of curves?

-The script explains the concept of a limit by describing how a sequence of jagged curves, each with a perimeter of 8, can be made to approximate a circle more closely with each iteration. It emphasizes that the limit of these curves is the actual circle, and the limit of their lengths is 8, highlighting the importance of rigor in mathematical proofs.

### What is the claim made by the second example in the script about the value of pi?

-The second example claims that pi is equal to 4, by using a square inscribed in a circle with radius 1 and arguing that the perimeter of the square is 8, thus incorrectly deducing that the circumference of the circle is also 8 and therefore pi equals 4.

### How does the script use the concept of congruence in geometry to prove that all triangles are isosceles?

-The script uses the concept of congruence by drawing perpendicular bisectors and angle bisectors to create triangles that are claimed to be congruent based on side-angle-side or angle-angle-side relations. This leads to the conclusion that two sides of the triangle are equal, thus making all triangles isosceles.

### What is the error in the 'all triangles are isosceles' proof?

-The error lies in the assumption that the angle bisector intersects the opposite side within the triangle. In reality, for many triangles, the intersection point sits outside the triangle, which invalidates the subsequent congruence claims and the conclusion that all triangles are isosceles.

### Why do visual proofs sometimes fail to provide accurate mathematical results?

-Visual proofs can fail because they may rely on incorrect assumptions, overlook hidden variables, or misinterpret geometric properties. They can also be misleading due to the limitations of human perception and the inability to accurately represent complex mathematical concepts through simple diagrams.

### What is the importance of rigor in mathematical proofs as illustrated by the examples in the script?

-Rigor in mathematical proofs is crucial to ensure accuracy and validity. It helps to identify and correct hidden assumptions, edge cases, and logical fallacies that can lead to incorrect conclusions, as demonstrated by the flawed proofs in the script.

### How does the script use the concept of Gaussian curvature to explain the limitations of flattening a sphere?

-The script mentions Gaussian curvature to explain that the geometry of a curved surface, like a sphere, is fundamentally different from flat space. Flattening a sphere inevitably leads to a loss of geometric information, which is why visual proofs that involve flattening curved surfaces can be misleading.

### What is the key takeaway from the script regarding the use of visual intuition in mathematics?

-The key takeaway is that while visual intuition can be a powerful tool for understanding complex mathematical concepts, it should not replace rigorous proof and critical thinking. One must always be vigilant for hidden assumptions and ensure that visual arguments are mathematically sound.

### How can the script's discussion on the approximation of areas under curves be related to the fundamentals of calculus?

-The script's discussion on approximating areas under curves with rectangles is directly related to the fundamentals of calculus, specifically the concept of integration. It highlights the importance of understanding the error in approximations and the need for rigorous limits to ensure that the sum of the areas of the rectangles approaches the actual area under the curve.

### Outlines

### 🔍 Exploring Fake Mathematical Proofs

The paragraph discusses three progressively subtler fake proofs and their implications for understanding mathematics. It begins with a flawed proof for the surface area of a sphere, which incorrectly assumes that subdividing the sphere into vertical slices and unraveling them would lead to a rectangle whose area can be calculated. This approach mistakenly concludes the area to be pi squared times r squared, rather than the correct 4 pi r squared. The paragraph then introduces a proof that pi equals 4, starting with a circle inscribed in a square and using a sequence of curves to approximate the circle's circumference. The nuances of this example are explored, emphasizing the importance of understanding limits in sequences of curves. The paragraph concludes by highlighting the need for rigor and proof in mathematics, rather than relying solely on visual intuition.

### 🤔 The Pitfalls of Visual Intuition in Mathematics

This paragraph delves into the limitations of visual intuition by examining a false proof that all triangles are isosceles. It challenges the reader to identify the flaw in a step-by-step Euclidean-style proof that relies on drawing perpendicular bisectors and angle bisectors. The proof uses symmetry and congruence relations to incorrectly conclude that any triangle is equilateral. The paragraph explores the possibility that either all triangles are equilateral, Euclidean reasoning can lead to false results, or there is an error in the proof. It emphasizes the importance of critical thinking and the recognition of hidden assumptions and edge cases in mathematical proofs.

### 📏 The Geometry of Curved Surfaces and Limiting Arguments

The paragraph contrasts the incorrect sphere surface area proof with a valid proof for the area of a circle using pizza wedges. It explains that the sphere proof fails because flattening the spherical wedges distorts their shape, leading to an incorrect calculation of area. The paragraph clarifies that the width of a spherical wedge does not grow linearly but follows a sine curve, causing an overlap when wedges are interlaced. It uses a rearrangement puzzle to illustrate how area can seemingly appear or disappear due to slight geometric distortions. The paragraph concludes by noting the fundamental difference between the geometry of curved surfaces and flat space, introducing the concept of Gaussian curvature.

### 🧩 The Subtlety of Limiting Arguments and the Importance of Precision

This paragraph focuses on the subtlety of limiting arguments, particularly in calculus, using the example of jagged curves approximating a smooth circle. It points out that the limit of the lengths of the curves is not necessarily the length of the limit of the curves, as demonstrated by the incorrect proof that pi equals 4. The paragraph emphasizes the need for rigor in limiting arguments and the importance of being explicit about errors in approximations. It also revisits the isosceles triangle proof, showing that a more careful construction of the angle bisector reveals the proof's flaw. The paragraph concludes by stressing the indispensable role of critical thinking and the avoidance of hidden assumptions in mathematical reasoning.

### Mindmap

### Keywords

### 💡Visual Proofs

### 💡Surface Area of a Sphere

### 💡Limit

### 💡Pi (π)

### 💡Euclidean Proof

### 💡Isosceles Triangle

### 💡Congruence

### 💡Perimeter

### 💡Gaussian Curvature

### 💡Rigor

### 💡Hidden Assumptions

### Highlights

Three fake proofs are presented to illustrate subtleties in mathematical reasoning.

The first proof incorrectly calculates the surface area of a sphere by subdividing it into vertical slices.

An unraveled approximation of the sphere's shape leads to a mistaken formula for surface area.

The true surface area of a sphere is revealed to be 4 pi r squared, contrasting the incorrect proof.

A visual argument is presented that pi equals 4 by approximating a circle with a square and iterative curves.

The perimeter of the square and the iterative curves is consistently 8, suggesting a false value for pi.

The importance of rigorous mathematical proofs over visual intuition is emphasized.

A Euclidean-style proof is introduced claiming all triangles are isosceles, which is inherently false.

The proof uses perpendicular bisectors and angle bisectors to suggest congruent triangles.

A series of congruent triangles are incorrectly used to deduce that all sides of a triangle are equal.

The proof concludes incorrectly that all triangles are equilateral due to a flawed assumption.

The difference between valid and invalid visual proofs in geometry is explored.

The geometry of a sphere's curved surface versus flat space is discussed in relation to Gaussian curvature.

Limiting arguments in calculus are highlighted as a point of potential error without proper rigor.

The importance of considering the limit of lengths versus the length of limits in mathematical proofs.

A rearrangement puzzle is used to illustrate how area can seemingly appear or disappear through manipulation.

The final example's flaw is pinpointed in the assumption that a point lies between two others on a line.

The necessity of critical thinking and avoiding hidden assumptions in mathematical proofs is stressed.